Thanks for all fast answers! I really apriciate it!
I should have said that my question regarded road-bikes but it seams that most of you have understood that.

Thanks for the many alternative tips (especially to lock at the fork-ends), I'll remember that! If you would like to you could post other quality-indicators. I know that best way would be to know everything about every label and model but I'm just a teenage-newbie who don't know that much.

Thx anyway!