i really like this girl and i know her really well, but i come off as being lame and childish and immature. is there a way i can change the way she sees me?

Only by changing what she sees. Lame and immature can be excused as being nervous around her. Childish is another matter. What are you doing that makes her see you that way?

there are several factors to this bud. Give me the details on the situation and I can give you an answer.

well, sometimes i'll call her and won't know what to say after awhile and there will be this awkward silence and i'll say something lame. and sometimes she'll think my humor is repetitive or stupid.

If you know her well, you should know some of her interests. Look into a couple that seem like something that would interest you. Read about them, learn more about them. If it is an activity, get involved. Doing this will help with the awkward silences, but remember that most conversations between friends will have an awkward silence at times. Often, that means it is time to end that conversation. It is much better to end a conversation too soon that too late.

Regarding humor - Don't force it. Humor really isn't jokes. Humor is a natural comment that follows something seen or talked about. Don't try to be funny. If humor is forced, it will sound false and strained. If a funny thought comes to mind that fits into the convesation, tell her. If none do, don't force one into the conversation; it won't be funny.

What's something that you say to her that sounds lame?

Yeah, I bet you just need to develop a better personal connection with her and get familiar with talking to her. It'll help you be more comfortable around her. Dorian's suggestion seems like a good place to start. Like your humor, stay calm and natural in your interactions.