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Forum Hacking
As much as i do not agree with your current set goal. I will; say this, person trying to hack forums. Learn about what hacking is. Try playing the war game here, the challanges are pretty good and they have a part on the message board for helps. No one will give you a direct answer(if they do they are defying the entire purpose of this site and others like it) But they will point you in the right direction.
- search for exploits on that forums on BugTraq etc
- cookie stealing
- SQL injection
- remote code execution
- SE the admin
- find admin hash on other site
- perform cross server hack to view the config.php file
Of course these arent the only ways, there are many more.
I suggest you do your own research into what i have posted above. People wont want to spoon feed you.