I recently bought a Second hand BlackBerry Bold 9000.
It is all in perfect working condition. The previous owner cleared everything.
The thing I am confused on, is the BlackBerry IM.
It is set up with his/her BlackBerry email. I wish to make my own, and use it on there.
How do I make my own, and get rid of the Previous owners email?
Thanks in advance! ^w^

Obviously the previous owner didn't "clear everything". You need to have them contact their carrier and CANCEL their BIS account. That will release the PIN and allow you to set up your email, providing you have a BB data plan active on your account.

By clear everything, I meant such as their SMS, Contacts, Photos etc.
I bought this from a Pawn shop, it is kept annonymous if they wish. So I can't get them to contact their carrier.
It is locked to Telstra. Would it be an option to go into Telstra, and see if they could do it somehow, otherwise?

You could try but doubtful that they will do it. You might find a CSR willing to help, but be patient and be nice. Do you have a proof of purchase?

Yes, I kept my receipt (That was actually just incase it didn't work :P)
And sorry I'm not familiar with it, but what is a CSR?

Customer Service Representative. It often takes considerable patience when dealing with them. Charm and kindness are your best weapons.