Volunteer as a game designer

Do you know guys, some sites where can I volunteer as a game designer? Or maybe some paid job, but I don't have enough experience to show off?

Well, that's dead simple: look for game publishers or larger development studios in your area and send them your profile and tell them your interest into learning game design. Make sure that the  studios do the games you like and prefer. It's not complicated and they don't bite. Trust me

In the US, we call these internships. You generally need to be attending college and doing work in a relevant field to apply for them since the spots are pretty competitive, and more likely than not,  you'll be going into a game testing spot rather than a coveted game design or level design position unless it's a really small/unknown place.

The best way to snag any kind of position is to build a portfolio of games you've already created in hopes that that will set you apart from the pack.

This is a strong example of an online portfolio.
http://simpanen.carbonmade.com/