Well, you should first play smaller games at the 500 - 1000 pt level. Then move to larger games at the 1500 pt level.
For the latter level, you need 2x10 Tactical Marines with two special weapons each, and 10 Scouts with 5 sniper rifles, missile launcher. This gives you a decent core of 3-6 scoring units.
SM can be played footslogging or mechanized depending on your playing style.
A mech army should field Rhinos or Razorbacks, while a footslogging army needs some good shooty units.
I suggest looking for army lists at this forum and then coming back.