Noobish question...

Do I have to choose other class after 10th lvl?

You have to choose the 2nd profession before leaving pre-searing and going to the REAL game in post-searing

i think you need the secondary to go through the academy but i dont know of any 2nd requirement in presear at any level

yeap you need a second profess to enter the academy

I have second question...
When i'm second class (f.e: ranger) can I give attribute points to Warrior attributes?

You can always give points to any attributes you have.

You have a Primary and a Secondary Character. I take it you are a Warrior/Ranger. Your primary decides your armor and most your attributes. You can put points into either your Warrior or your Ranger half.

Can I play PvP using my character from normal game?

Yes, there are arenas in the towns that you can enter with our normal PvE character.