well, im going to start by saying that, EVERYTHING comes with time and experience with the game, nothing can beat that.
now with that in mind, im going to go over a few build orders for SM. im not going to say "you must get this squad" because it largely depends on the matchup, map and what commander you are.
buy a tactical marine squad instantly, there is never a situation where you wont need them. your tactical marines are your bread and butter unit and will be doing the most dps in T1. they also have a 150% capping speed.
now if you're playing 1v1s, you'll need to buy a 2nd scout squad after your tac purchase, so that you can run around the map and cap things with them. as we're on the topic of scouts i'll go more in depth.
the shotgun upgrade for scouts is really nice, it brings their dps up to respectable levels and gives them a knockback ability, this ability will be how you control melee squads in T1.
the sargent gives you a really nice frag grenade, really nice for flushing units out of cover and buildings, and if they dont dodge it, it'll do a lot of damage. the sargent is also a detector (can see nearby infiltrated squads) and adds durability to the squad. if you can learn to be good with grenades, its a really good upgrade. it is however expensive at 25 power, so it needs to pay off.
snipers are your garrison counter from scouts and are just good at picking off models from long range. you do give up that shotgun blast though.
elite training is a no brainer, does what it says on the tin. infiltration and health regen. can be used to sneakily decap points or sneak up to your enemies army if they dont have a detector.
Assault marines are your setup team counter, you can jump them on to entrenched positions and to tie up units.
devastators are your setup team, can be used to suppress and thus counter ranged blobs, just be careful of setup team counters. each race has one. in T2 they can get a lascannon which is a vehicle snare and the mainstay of SM AV.
there is something called the 1-1-1 build which is one tac+one dev+one asm squad. its versatile and can be used to tackly most T1 compostions. but if you spend power on your scouts/commander/tacs then you might be a bit behind in tech. this however is all up to you to decide if you really need an upgrade and if you can deal with the inevitable quick vehicle from your opponent.
once T1 is over, its largely reaction based with what you get, dread for big blobs of melee, librarian to support devs, pdevs if you're floating req or want some ranged disruption etc etc. like i said it will all come with time and experience.
ghorms suggestion of watching casts and streams is a sound one, if you're someone who learns from watching other people play that is.
here are a few casters and streamers: 
https://www.youtube.com/user/DoWReplaysNet - indrid, one of the biggest casters. i learnt a lot from him when i was starting out.
https://www.youtube.com/user/RadicallyDreaming - Maestro
streamers: 
www.twitch.tv/venkitsune - myself, i stream every day almost.
http://www.twitch.tv/megametzelfleischer - max power, map designer and streamer.
you can join the elite steam group for announcements when someone starts streaming: 
http://steamcommunity.com/groups/DoW2EliteMod