I start with 2 pictures:
Open the picture of the background. Select the girl and move her to the new background using the Move tool (V). I used the free transform tool (Cmd T) to scale it.
Next I created a shadow around the girl to make it more realistic. Double click on the girl in the layer panel. I chose Drop shadow.
These are the values I used for the drop shadow.
Last thing to do is to blur the edges of the selection of the girl with the blur tool. Merge the layers (Cmd E) down and add some blur around the girl.
This are the basic steps to add a subject to another background. Go on, give it a go!
By the way, the picture I used comes from pixabay.com