when i click pvp arena all i see is a green field with my character somewhere in it, and a door wich i can double click to leave

how do i fight anything?

PvP arena is for fighting other players, usually you will want to organize a battle first before just entering.

Click on Catacombs to fight monsters. You need to "Engage" by clicking the appropriate button or hit the G key. You can then move around with the W,A,S,D keys or arrow keys, and click mobs to attack them.

You should also read through the LS Portal if you are serious about playing, it has all this info and a lot more that starting players should just look though. On the left side, under Help and Guides, read each link when you have a few minutes time.

LS Portal

right thanks, unfortunately, my brain is way to damaged to read big pieces of text and save them into my memory, lol

That's why it's just a click away, there for when you need a reference.