I currently have 3 armies, and whilst it is damn bloody expensive, if that is your hobby - and particularly - if you have friends who do it with you - it's well worth the price. Takes up time, gives you a few cool painting skills, you get to work with your imagination, and then there's the delicious violence.
Plus now whenever I'm not training/teaching or looking for a stable income, I'm at the local GW store.
Personal advice?
Don't do Space Marines unless you really REALLY like the hauty always gotta be good guys kind of thing. After a while (imho) Space Marines get really stale really quickly.
I know that's hypocritical considering one of my armies is Black Templars, but I'm customizing the hell out of them.
I also have Chaos Marines and Dark Eldar...so there's always some variation.
To start off, if you can - go to a GW store. Ask them! Most of the time you'll hear "buy Assault on Black Reach. SM + Orkz + Rulebook = Gives you a taste" but if you know you aren't interested in those armies, or you want something you can fiddle with, then look around. Find models you like, buy a troops choice and build them. Go from there, and don't worry about making stupid decisions.
This is a personal hobby that is damn expensive, very time consuming and based around your own interests.
The only way you can really stuff up is if you have hundreds' at the ready and you buy a bunch of unnecessary crap (like tanks, bikes, greater daemons, knights etc) for your force, because then you can't play

Like the guys above have said, HQ + 2 Troops. Hell, rummage through the GW site. That'll give you an idea.