Quicktime Pro and adding backgrounds

I have a background that I would like to apply to my movie/video. How do I got about this?

1) Open the Movie in QT 7 Pro.
2) Open the "background" in QT7 Pro.
3) Use Command-A and Command -C to select the background and copy it to memory.
4) Close the "background" player.
5) If not already active, reselect the player window containing the movie and use Command-A to select the entire movie.
6) Use the "Add to Selection & Scale" Edit option to apply the background to the entire movie.
7) If background is "above" movie, select the background or movie track and adjust its layer so that the movie layer is displayed above the background layer.
8) Adjust offset and/or size values for the video track so that it displays where you want on the background layer.
9) Save the finished file.

The "video" would use layer -1 (lower numbers are closer to the front).
The background would be layer 1.
You also need to adjust the "offset values" for the video track. Default is 0,0 (upper left corner) so the video track would need to be changed to help "center" it over the background.

how would I go about getting rid of my original background

To delete a background track, select the track and press the "Delete" button.

how do I just extract a new background and put me in front of it

You would normally create backgrounds in a graphic application and add them to the movie file. Backgrounds can be scaled to the length of the movie or scaled to fit behind selected portions of the movie. Multiple backgrounds can be be overlapped. In addition, alpha channel objects (e.g., a picture of yourself in its own transparent background created in Photoshop) can be sized and/or layered over the background in one location with your movie layer sized and/or offset layered in another. An MOV file can have up to 99 tracks which can be audio, video, graphic, text, chapter, etc.

I keep layering and centering but I still have my original background.

Not sure what you want to do here. You can overlay a second (or more) background over any portion of the first, only add the first background to a selected portion of the movie, or, as mentioned above, delete an entire background track if you find you no longer want it.

Not sure this will help your current questions, but here are some old sample tutorials demonstrating the basics of layering video over a static background and layering segments of one movie over a second movie used as the background. These "Quickie" tutorials were created for other QT users who had similar questions in the past.

Layering a Movie Over a Static Background
Layering a Movie Over a Movie