The community thinks Irelia is better, because the best teams in the world sometimes play her but rarely play Kennen.
This is a terrible reason to suggest one champion over another to a player who is not on one of the best teams in the world.
It is much more important to get a champion who works well for you instead of one that is approved by the community. Just because people think one champion is better than the other doesn't mean you'll play them better.
If you're level 13, virtually none of the champions' strengths and weaknesses (e.g. counterpicks, performance in the current meta) apply to you AT ALL. Pick whoever you want.
Also, if you're level 13, right around now is a good time to stop spending IP so that you can afford to go on a spending spree at level 20 when tier 3 runes unlock. You'll want at least 10,000 IP for that.