In low level, pre level 30 it is usually 2-1-2, 2 top, 1 mid, 2 bot. Don't really worry who goes where at first, but usually it is 1-1-1-2:

1 top, this is usally a bruiser or at least someone that can stay alive on their own, 1 jungle, this is usually either a bruiser, a tank, but also other special cases (best not to think too much about this role for now), imprtant to the role is being able to gank and clear jungle without dying 1 mid, which is a mage or assassin, if it is a magic (AP) champion then top should be a physical (AD) champion or the other way around to keep damage output balanced. 2 bot, this is the AD-Carry or Marksman, he is the main source of the physical ranged damage of the team, strongest at pushing, super squichy, gets all farm, has a huge ego, needs to be protected, and his "bitch" the support whos job it is to ward, protect and in general help the whole team. supports are usually a few mages with shields or heals etc. or tanks with lots of crowd control.

The main damage are magic (AP) damage from mid and physical (AD) damage from bot. You play 2 bot because you can control dragon better that way. You play the magic carry mid because it the shortest (savest) lane and it is easy to roam to other lanes and help out, since they usually have huge burst and some crowd control, so they can gank. The other 3 are usually a mix of damage, tankiness and crowdcontrol. To answer your example you do not play Cho'gath bot because he is melee and would constantly get harrasst by the long ranged marksman in bot lane.

I hope this helps, if you don't understand some terms just ask.