depends what ur playing. if ur playing a pubgame, u can choose to get one of the pub stompers. tryndamere and xin are both good right click to win pubstompers.
if ur a bit more pro and not somewhat retarded. you should know this
top lane : fighter/bruiser (solo)
mid lane: ap carry (solo)
jungle: jungler (solo)
bot lane: support (DO NOT STEAL CS)
ad carry (TAKE CS)
other simple things like dont play shitty champs should be obvious.
here are some recommemded champs for lanes
top: jax,renekton,irelia,nasus,trynd,skarner,xin,nidale e, others
mid, brand,lux,annie,sion,kass,cass,malz,gragas( <- pro) other shit like that
jungler: nunu,amumu,udyr,riven,ww,gp,fiddle, shit like that
bot: AD carry: tristana,vayne,corki,caitlyn,graves,ashe, thats pritty much it
support: sona,soraka,ali,taric,janna shit like that. (remember dont steal ur carrys cs) (and also take cv) (and remember to cv enemy spawn at start)