I'm searching for an arena booster but didnt found where to find it. please help me. thanks

In the PvP Arena, which is not yet released. It will be coming in August: https://pso2.com/news/announcements/roadmap

If you only need the boosters, I wouldn't worry about doing well. Just Point and Click Adventure on enemy players. That's pretty much all you can really do if you aren't used to most weapons. (As weapons come in the form of pickups on the ground, and are randomized.)

I see. thanks man.