Cthulhu's advice is in general spot on, but.
Be advised that while this is kind of set in stone for high Elo games (or was, as the Moscow5 games showed pretty graphically - AD kennen top, yeah), in low Elo feel free to break the meta in any way you want - play the champion you feel are best with, rather than the champion the metagame forces you to play. Even if you are forced to play support you still don't need to play a passive support - go Leona/Alistair/Gangplank/Blitzcrank and make a nice kill-lane. Even the passive supports (Soraka, Sona) benefit more from aggresive play - zone the enemy team, harass with infuse/whatever sona's Q is, set up kills. The current 0 CS support 1/1/2 meta is why I don't play SR at all nowadays - it's too passive and boring, emphasizing farming and not dangerous kill-or-die gambles.
Also bot is considered the important lane because the dragon's there, and it's important to try and control it, or at least prevent the enemy from controlling it, while top is where Baron is, which is a late-game objective requiring the entire team and most often in low Elo games not worth it (because the notion is "one of them died let's baron!" while it should be "one of them died lets push mid/top/bot and take a turret!").
Top is populated by bruisers with sustain because the jungler prefers ganking bot since 1. dragon and 2. easier to get a kill on the enemy support.
Also Talon generally goes mid to counter enemy AP casters, though I've never been good at that ._.
edit : tank is generally a term that is considered wrong - what you need is good initiation, someone to start a fight in your favour. It could be a tanky bruiser type like Rammus (who is usually jungle), Alistar (who is a support but can also jungle), but also things like Ashe arrow, Wukong ult (who is a top bruiser), Anivia wall and stuff like that. So Rammus and Amumu go jungle, Alistar goes bot, Wukong, Gangplank and stuff go top.
Further also I don't play SR and only watch streams and read thread so I might be talking out of the proverbial ass.