Toribash
Depends on which position you are going for. Normally, if one need to ask how to become one, he is not a preferable choice.
If you make a name for yourself you're generally asked, or you might feel as though some people may know you well enough that you might express interest in becoming a forum leader. It's good to have a history that's relevant to the position you want to apply for, as there's more than one type of forum leader, although Hero is right. Generally, if you have to ask how, there's a reason you don't know.

Best of luck though!
collect snots from the nose