If its only for educational purposes, you might want to post a link and discribe what you did. But I'm quite sure that its either already explained in the links I gave you or not legal to test it where you would like to test it.
I have lerned most of the thinks I know by solving challenges and by programming. If you program a simple forum and try to abuse it, you can see where the problems are. Then take the other part and try to make it impossible to use these vulnerabilities you've just used.
This approach takes a lot of time, a lot of thinking, reading and programing.
You might also want to take a look at some well known forum systems like
phpBB. Their source code will be much longer than a simple forum you could write. Try to understand phpBB and some parts of the code which are relevant for security. Eventually they have comments like \"this is against XSS\", I don't know.
You could also look at the phpBB
bug tracker. They might have some security vulnerabilities which they have already fixed there. So you could try to understand how someone could have used this vulnerability.