Place the background photo in a layer below all the other layers and perhaps add a blur or other filter.
What is your start image? Is it an image that fills the entire frame, or an object with a transparent background? If the former, then you will need to select the area you want to retain, and cop it to a new layer. Then place the blurred layer beneath it in the layer stack. So Slections is what you might need to be looking for on YouTube
You will gennerally find that the official help pages are as good or better than third party offerings on YopuTube.
Background in Photoshop is done by layers. You have foreground layers and the last layer is the background layer.
Foreground layers need some kind of transparency to let the background shine through. That’s it...
Look here for more technical instructions: Learning layer basics in Photoshop
You may also look at Adobe TV (very good videos) and Youtube (you need to distinguish the good and bad videos...) for layers and compositions. THere are a lot of possibilities. You can‘t enumerate them in a thread like this.
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Many great suggestions already posted.
One more item to add, in order to have a layer below your current background layer, you'll need to change the current Background layer from the name Background to something else. Simply double click on the Background layer, and rename.