how do i know which champion plays jungle,mid,top,support or bot?

when you are new the meta dont matter, its not til about lvl 15ish people understand roles and champs in said roles.

I guess any champion with a strong early game can jungle. Everything else is up to you, nobody can force you not to go Ahri support or heimerdinger top, they're all just lanes. The only difference is that mid is easier to gank and it has different matchups.

Yes it matters. I would suggest just google the champion you want to play to know about it.

None of it really matters until people figure out what they are even doing and begin functioning as a team.

Top and bot used to be identical lanes, until the recent changes to early-game turret shields. What lane a champion is played it is not really pre-decided, you can play anything anywhere if it is in a situation where it will work, and you will often see champions switch preferred lanes every season or after big patches; as an example graves used to be an ADC, now he is almost exclusively jungle.

Marksman usually play ADC, tanks usually play top/jg, if the look magical that's either mid or support. If they look like assassins that is probably mid. Supports are probably the hardest to point out if you don't know.

there isnt a distinct sign to tell you which champion goes where. Players find by experience and practice whether champ works for this or that role and then it is spread about the community which shapes the "meta". With time you will see for yourself it matters alot and in more competitive games it plays a major role. Its a combination of small things like how well a champ can farm creeps, how much sustain it has in the jungle, does it have slows/CC tools, can it take high amount of dmg, can it achieve high burst dmg, does it require items to be efficient, or is it fine even without good gold income. All that plays a major role how you will perform at the end.

try this site it tells you how good a champ does in a certain role http://champion.gg/ and if your not sure just make a custom game and try it out