Hack what!?!?!? Ask only direct questions here! It's like I'd ask "Do you know how to write?" And then I would give you some chinese text LOL
I hope you know that these actions are illegal! You must inform the owner about your actions and he has to agree. Nevertheless, you can accomplish this by getting into some SQL database, you can pull out the passwords and usernames out there. Or if you want to crack certain account, you can use brutefrce method. But remember these actions are illegal!
Steps; 1st - Learn proper grammer 2nd - Read one of the threads about ethical hacking and what is involved 3rd - Google 4th - Don't expect someone just to say "hey here's how to do it, let me hold your hand" ... hacking is a mainly self-taught art, it takes time.