i have finished all my classes and now I should start my master thesis. But first of all, I need to write an email to prof. and ask him about the available topic for the master thesis. How to write this email? does anyone have experience with that?
This is better handled in person, rather than by email. You will get a more useful answer and be able to explore options. I would only use email if there were no face-to-face options available. Bring a printed CV to any such meeting in case the professor wants more information on your background. Mention in the CV any projects you have done. And make sure you know at least a bit about the professor's research interests before you meet.
But if you need an email structure it so that you give some information, but don't make it difficult to read or necessary to read everything. First ask, then say something about yourself and your background and interests. Try to keep it short and it will be more likely to be read. Anything very long (like a CV) can be an attachment, but probably isn't necessary for an initial contact. I assume that the professor will, if interested, ask for a face to face meeting in most cases. Again, make sure you have an understanding of the research interests of the professor and that they match your own.