First, a rundown of the strengths of the individual squads.
Terminator Squads: These squads have the benefit of Tactical Dreadnought Armour, which gives them a 3+ save on 2d6, all ranged weapons have targeters, which means that a Terminator will fire with an effective BS of 6. They also have the ability to use such weapons as the Assault Cannon in a move AND fire mode. They have the stormbolter, which is a sustained fire dice weapon, and if they do not move, can rapid-fire. They have very powerful hand to hand weapons, and can be teleported.
Veteran Space Marine Squads: This is a Tactical squad with +1 WS, BS, and Ld. This squad can lay down a large amount of firepower accurately. You can give the squad one heavy weapon and one special weapon, this is a very versatile squad.
Assault Squad: Hand to hand weapons and jump packs, this squad excels in hand to hand, but is weak on firepower. It can also become quite expensive pointswise if you aren't careful.
Tactical Squad: Versatile firepower.
Devastator Squad: Basically a Tactical Squad with four heavy weapons, use this squad for taking out tanks, characters, and for laying down blanketing firepower against armies that have large numbers.
Scout Squad: Infiltration with versatility of armament, good for setting up ambushes and to slow down your opponent's advance. If your opponent is careless in their setup, Scouts can be used to ambush characters (ie psykers) and support weapons. Don't forget Scouts can have a heavy weapon or a special weapon.
Bike Squadrons: These are great for making flanking hit and run attacks. They can even be armed to take out tanks in close combat.
Support
Tarantula: A very versatile anti-grav platform that can have several different weapons installations. Most of these will be in the twin-linked form, except for the multi-melta.
Rapier Laser Destoyer: Basically a tank killer.
Whirlwind: Great against enemies that have weak armour saves. Can fire indirect, template weapon, not much in the way of close-up defensive weapons though.
Land Raider: Massive troop carrying tank armed with twin lascannons x2, and twin heavy bolters. The main major problem, it attracts the enemy's fire like honey attracts flies.
Predator: The most versatile tank there is, everything from a 90-point autocannon-only vehicle through a 210-point vehicle that has four lascannons, and everything in-between. You can customize its armament to suit any enemy you might be fighting.
Land Speeder: Fast, can make pop-up attacks, but is better used for flanking runs, great for making crispy critters out of armies that like to hide in woods, the heavy flamer could care less about cover.
Rhino: With a little imagination, this 50-point vehicle can be used for a myriad of tasks. A very versatile vehicle, and points well-spent.
Razorback: Good firepower with lascannon and twin plasma guns, and can carry a combat squad. Great vehicle for supporting the infantry advance.
Now for some specifics on how to use the army.
Always remember that this army's strength is in its versatile firepower, and its staying power, use these two things to build your tactics.
Vs. Orks: Use a lot of template weapons, and sustained fire dice weapons for your Devastators and Tacticals heavy weapons. Emphasise your ability to rapid-fire, and create killing fields by deploying in such a way that several squads can cover each other and the approaches to your battle line with converging firepower. Cut down as many greenskins as you can before they can make contact. For your Terminators, assault cannon, heavy flamer, or Cyclone Missile Launcher, and make sure the other Termies have stormbolters. The lightning claw and thunder hammer will not help you againts this army. A Tactical squad or combat squad with jump packs to harass and attack tactical areas and/or characters is a good idea. Use bikes for flanking attacks, and to hit and run the Orks' light vehicles. Your tanks should be armed to take out large amounts of enemy troops, the autocannon is very good against Orks, as it has the strength to take out their vehicles and sustained fire dice.
Vs. Eldar: Most of what I said vs. Orks also works here, but you will need better armour penetration for taking out such things as Dreads. Again, let me stress, with a Codex Astartes chapter, firepower is your main strength, so take advantage of it.
Vs. Bugs (Tyranids): Use the same tactics as above, but use more flame weapons to give you large template weapons. Do not fight the Bugs hand to hand with this army. Only your Terminators can handle hand to hand against Tyranids. And against the Tyranids, give your Terminators good hand to hand weapons like chainfists, power fists, lightning claws, etc. Against this army, I usually use two Terminator squads when playing a Codex Astartes army, I will arm one for firepower and the other I will give a heavy flamer and lightning claws. I use these to take out the Bugs' heavy hand to hand troops. Make sure to give characters things like bionic eyes and sensors to help detecting and/or spotting the Bugs' hidden troops (ie Lictors). Always cover your squads with other squads when fighting against this opponent. Take out the Tyranid Warriors, Hive Tyrants, Carnifexes, etc. early to mess up the Hive Mind, this leaves the lesser Bugs with poor Ld and easily broken. Let me say this again, use your firepower! Don't forget rapid-fire. One little thing, with a heavy flamer you don't have to see what you're shooting at, as you do not make a BS roll, so it is good for taking out hidden troops.
Vs. Other Marines: In this situation, you must use your firepower, support weapons, and vehicles to support each other and make sure that all of your support weapons and vehicles are covered by your infantry to keep the specialized troops of the other Marine chapters from being used to their fullest ability. Do not go out and try to fight hand to hand versus Blood Angels, Space Wolves, etc., do what your army does best, shoot them! Against Space Marines, go to the heavier armour save minuses-type weapons (ie lascannons, heavy plasmas, autocannons, etc.). Give yourself the weapons to take out the inevitable Terminators and vehicles you will be facing, including Dreads.
Vs. Chaos Marines: See above entry for Marines. You may wish to give all your psykers Force weapons.
Vs. Chaos Cult Armies: Use Vs. Ork tactics. Just beef up your psykers and give them Force weapons.
Vs. Daemon World Armies: Vs. Ork tactics with Grey Knights and Inquisitors. Avoid hand to hand combat with Chaos Warriors, except with your Termies and Assault troops. The same goes for the prior two armies.
In General for Fighting ALL Chaos Armies: Avoid hand to hand with any of the Berserkers, Khorne Warriors, Chaos Warriors, ANY Nurgle troops, except with Terminators, Grey Knights, etc. Try to kill off as many of his Chaos Marked troops as possible to limit his summoning.
Vs. Imperial Guard: You must have both weapons to take out hordes of lightly armoured troops and weapons to kill his heavy vehicles with. This means lascannons or special armed Assault troops and/or Bike Squadrons designed for killing armour in close assault! Rapid fire the infantry to death (literally!), take out his heavy weapons teams. Kill the IG's leaders and you've gone a long way to defeating the IG, as they are more dependent on their leaders than most armies, except the Bugs.
Let me go over this:
Use rapid-fire. Set up killing zones. Concentrate fire of more than one squad on an enemy until you have eliminated that unit before moving on to another target. Firepower is your main strength, and concentrating that firepower on one target multiplies its effect. Splitting it amongst many targets weakens the effect of your firepower. Use blind grenades, etc., to slow down and block areas of your opponent's line (40K is basically a skirmish game between opposing forces made up of small units, so when I say line I mean basically a skirmish line of small squads, not the massed line of WHFB) while concentrating firepower on other parts, this is one of those places where the lowly Rhino comes in handy for firing blind grenades.There are several psychic powers that can also be useful for slowing down the enemy so that you can concentrate your firepower. Think about gaining an advantage based on the strengths of the Codex armies and covering its weaknesses (lack of hand to hand) at all times. As my instructor in command college liked to say, "There is no such thing as a dirty trick in tactics. Gaining an advantage for your army is the most important duty of any leader." Don't break the rules, that's not what I'm saying. What I'm saying is, be creative in your tactics, and take advantage of every strength your army has and every weakness your opponent has, this is just good tactics. The Space Marines have some very good vehicles and very versatile vehicles, two cheap vehicles are usually much better than one expensive one. Think about this when purchasing your vehicles, and think about the multi tasks it will be asked to do. Remember, you won't always be just fighting other vehicles. and remember, the worst enemy a vehicle has isn't another vehicle, but infantry. The Space Marine Dreadnoughts are also very effective. With this army I especially like the Devastator Dread with twin-lascannon and missile launcher and/or the Tactical Dread. Remember, it also goes with your Dreads, use your firepower, this is not an assault army.
Don't forget the importance of Chaplains and Apothecaries to your army, the more troops you can keep fighting, the better off you are. Also, in a firepower battle, Techmarines become even more important to keep your weapons operating.
Find good tactical deployment locations that give you cover and advantageous firing positions from which several squads can fire at the same area and can cover a large percentage of the battlefield. Set up your killing fields and then use them to your best advantage.
Do not make frontal assaults with your bikes and attack bikes, they are much better used popping out from behind cover to make hit and run attacks on your enemy's flanks.