Trevor and JJMack have provided you with a lot of useful tips.
The one thing that I would reenforce is the use of Layer Masks, for your Composites. That will allow you to do non-destructive editing, and you can go back years later, and make changes - IF you Save your Layered PSD file.
For doing what I think that you are working on, I will make a rough Selection of the Image to be Composited, and drag it, or Copy/Paste it into my background Image, and then create a fine-tuned Layer Mask for it. Note: it will come in as a Layer, which is what you want.
The Help File has full details on both Layers and on Layer Masks. I would definitely read both sections. Every moment spent with those two subjects, will pay dividends later.
Good luck,
Hunt
