Champion Help!!!

Alright, so I started playing league about 6 days ago and am currently level 8. I love using Yi and use him whenever I can. When I can't use him, I use Ashe. When I play with my friends though, somebody already uses Yi and I can't use Ashe since someone already uses Jinx for bot. I know Ashe can go mid but it can be pretty hard and apparently Ashe isn't for top. I need to find another champion to use that is somewhat easy to use and powerful. Plus, I probably should know how to play more than just 2 champions.

The current champions I have in mind are... -Tryndamere (heard he is pretty powerful) -Pantheon (looks pretty cool XD) -Katerina (very OP) -Jax (heard he's good at solo laning)

Can any of y'all be willing to give me any suggestions on who I should use and why? It doesn't have to be out of my little list but am mainly looking for either a fighter or assassin champion.

Thanks guys!