Since I started to play I tried to get the Riot Girl Tristana skin I have followed all the steps, but I can't get it in the game.

I was able to get the Allistar Skin (suscribing to youtube), can you help me?

I had an issue getting it as well. If you've got your facebook set to always have you logged in, that's likely your issue. Log out of face book, then go through the steps to get Riot Girl Tristana(which will make you log into FB during the process). That made it work for me.