Once you hit about level 20, you should know how to play one of every role. The roles are Carry, Mage, Tank, Offtank, Support, and Assassin. Supports are almost never seen in low-level play because it's all about damage. You might see a kayle, but she's hardly support anymore due to her massive heal nerf. Jebus made a guide to answer this specific question.
http://www.mobafire.com/league-of-legends/build/5-champs-you-should-know-how-to-play-aka-who-do-i-buy-next-70445
Read that guide. It will teach you a thing or two about team composition.
http://www.mobafire.com/league-of-legends/build/5-champs-you-should-know-how-to-play-aka-who-do-i-buy-next-70445
Read that guide. It will teach you a thing or two about team composition.

Special thanks to Keondre for the sig!
Due to inactivity, I am no longer reviewing guides. Sorry guys :(
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Click +Rep if you thought I was helpful please! Also check out my Ryze build!
I will Ryze to the occasion... hur hur