In my opinion, you have two main options:
The first: Work your whole body out. Do legs, bi's, tri's, lats, shoulders, traps, legs, abs, pecs, everything mate. Do this properly, and for the next couple/few days you should ache somewhat. This is good because you can normally do it all in like an hour and a half. You'll end up probably doing this about every 3 or 4 days once you're into it.
The second: Go for shorter sessions, working less muscles, with more frequent trips- This means say you do triceps and pecs on monday, tuesday you will do biceps and lats, wednesday you will do legs and shoulders, thursday you'll do abs and back..then friday you go back to triceps and pecs. Go 6 times a week.
As a rule of thumb though...if you leave the gym and you can physically lift ANY more weight without collapsing, you haven't done enough.
