Don't have too much faith in private browsing:
here,
here,
here and
here (just a few). The last one isn't about private browsing but nice to read to know what kind of stuff gets left behind.
massi, my advice to you is simple. If your little indiscretions are just that, your best bet is just to come clean. Sure, you can try to clean everything but does that not look a little incriminating? Think about it - someone goes to look and all your history is gone. Your parents will know that this was a poor attempt at covering up your tracks. Don't get into more trouble than you might already be.