Ok, so i'm fairly new to league, (level 18) and sometimes me and my friend like to 1v1 and of course, he always picks rengar and I lose horribly. In actual matches we always do about the same, if not, i do better. So i'm thinking that the campion thta im using is just a bad matchup. I main Talon but I also use Kha'Zix and Kassadin. Which one should I use against him?
get a new friend, rengar is meant to be a very strong early game assassin that snowballs harder then most other champion, against someone whose new he will just wreck. if you want a champion that can easily counter him one on one, pick 
as his passive will block rengar q and his stun will lock him into place while refreshing your passive. pantheon in general has the strongest early game in the game vs AD champions. he also very simple to play, its pretty hard to fuck up with him (bit mana dependent, he runs out quickly), since he has that good early game power you can bet that he doesn't do well late game.
if you really want to pick up one of those assassins, it really depends on where you like to play
jungle? kha zix definitely mid? talon
kassadin is kind of strange state, i wouldn't recommend him to anyone new.
Personally I would pick Bard or Tahm Kench against Rengar. I wouldn't pick them because I'm good with them, or because they are particularly good against Rengar, but because they are fun. If I'm 1v1ing someone and they pick a champion like Rengar I might as well make them have a bad time.
That said if you actually want to win instead of be a giant asshole walking around spamming laugh you should pick whichever champion you're best at. When faced with a bad matchup it's always best to go with a champion you know. Knowing a champion like the back of your hand can cover for some lack of knowledge you have about what to do in a particular matchup, and can assist with learning.
Kha' Zix is stronger than Rengar early game. And 1 more thing, I really hope your friend is just 1 v 1ing you for the purpose of learning rather than showing his dominance that he's a better player. Because you are new (level 18), I would highly focus on improving rather than 1 v 1ing with your friend.
stay away from bushes until lvl 9.
buy oracles lense. pop it when he ults (you hear the sound). ult yourself right when he jumps you, so he doesnt have a target to chain his abilities. at.
Reksai and shaco can work well.
Pick 
Rush 
You can poke the hell out of him pre-6 If he engages you after 6 you can just ult, stun and kill him. Zhonya should be used as a last resort.
As a rengar main who sits in plat currently.... I find jax, pantheon and Quinn to be rly rough match ups...
jax. he can't do anything to jax. pantheon also negates one auto and thats a lot of burst also he burst him down.
