I've looked at some people's cB pages and they are really awesome, I must say. But I've been looking for the area where you put the code in to add a background, music players, etc.

Can anyone help me?

I put mine in the footer.

For the profile? My account > customization

Click on "my account" and then go to "customize."

I have a question:
Whats the difference between the footer part an the header part?

i feel so stupid when it comes to HTML

Another question: How do I change my avatar? (Not my profile pic, but the avatar on my posts).

edit: NEVER MIND, found it.

The footer is the bottom of your profile and the header is the very top. Some people put banners in their header. A good example is this profile. Look at the top and very bottom of her layout.