The cool thing about dota is that you can play a few different styles that are all very viable. Generally you need a hard carry who will either solo mid or top(or bottom if you're radiant). They only get this solo lane if your team has a jungler like in LoL. If you dont have a solo lane for them you want to get a support hero to just sit in that lane and deny all of your creeps. They should let the carry take all last hits and pretty much just make sure he stays safe. This hero should buy the courier and constantly keep the river warded. For the middle champ, I personally like to use a ganker (Bloodseeker is one of the best for pubs ;D). Getting them up to level 6 and roaming as soon as possible is really great imo. For the other two you have a lot of options. You could grab another carry and support, carry and ganker, carry and lane pusher or dual lane pusher for extra fun. I really love playing a hard pushing style like venomancer broodmother offlane.

Hope i'm not totally wrong, GL HF!

edit: Another great way to learn team comps is to watch pro dota vods on joindota.com or something. They always explain why the teams make the bans and picks. I think tobi wan knows what he's talking about for the most part