Choose one as a background (this will be called background in the layers palette).
Go to 'window' tab (top horizontal bar) and make sure History palette and Layer palette are open
Top of tool bar * is for moving. Using this tool click on the picture you want to move into the background and click and drag on to background.
This picture is now in your layers palette
Now use the 'Quick Selection' tool (fourth down in tool bar - looks like a lipstick!). Sweep it over the object in the picture you want to select. Aim to select more than you will use and remember that at this stage this is quite rough, you will refine it later. If you select much too much then use the ALT key (on keyboard) and deselect what you don't want.
In Layer palette add layer mask:
From now on be careful what picture you are clicked on in the layer palette. For most purposes try to stay in the 'mask' .
In tool bar at the bottom there is a black and white colour option. Make sure you have pure black and pure white by clicking the Black and White (default color well) icon
Go up to Brush tool (make sure you have clicked on the mask you want to work on in the layers palette).
Flick between White (reveal) and black (hide) according to what you are doing.
Change the size of the brush in the top tool icon (horizontal).
SIZE
To change the size of anything go to edit - transform - scale.
