Hey Kharn, i'm a new relatively new player myself. The only advice i can give you is make a few army lists which scale in point size. When i bought my first load of stuff i made a 2000pt list before ordering. After that i scaled it up to 3000pts. In a few months i'll probably scale it up to 4000. The point being is that you should not buy a TON of models, some of which you may never use.
Get out there play some games, even proxy a few models so you can see if you like the unit/formation enough to buy the real thing. I really wish i could give you some better advice but sadly i play Imperial Guard (well...not sadly... i love the little guys). But you know what I mean.
The only Space Marine specific advice i can give you would be mobile infantry is where its at (in my opinion). As Hena said, the armour is points heavy. You have:
Tactical Squads - Firefight 4+, Close Combat 4+, 1X45cm Missile = Best basic inf in the game
Dev Squads - 2X Missile Launchers = Very good anti-tank. Load those guys in Razorbacks for some scary mobile firepower
Bikes - great response unit use them to help out assaults and grab objectives
Assault Squads - Close Combat 3+, Jumppacks and 30cm movement. Need i say more?
Plus with the They Shall Know No Fear rule the squads may seem small but they are running nowhere in a hurry.
I would suggest Thunderhawk or Dreadnaughts for close infantry support. Some whirlwinds/hunters would be good to. Hunters will give you some anti-air power
How many points do you have right now by the way?