As the background of what?
A photo composition, a collage, your computer desktop, a multi-layered file…?
Describe your goal as best you can, and give us detail over your setup, platform, OS and version of Photoshop, etc.
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Hope I understand your question well.
If you open a photo in PS it will automatically be the background.
If you place a photo on an existing background the best way to go is as follows: select the downmost layer and double-click on it, then click OK. Now it can be moved. Place it above the photo on layer two (click and drag). This way your photo will become the new downmost layer and so the new background.
Welcome to the forum.
I agree with Station_Two, about knowing exactly what you wish to do, what you desire to happen, and what you are working with, will be very important to helping you.
Harrieb has given you some tips on Background Layers, changing them to "regular" Layers, and Moving them in the Layer stack, and that might be what you needed to know - or maybe you are doing something else? If you can tell us more, and in more detail, someone will be able to help you.