How to combine two pictures to make one?

In Photoshop you open the 2 pictures your want to use - the background picture and the picture you want to put into it. Use the quick selection or lasso tool to select the part of the picture you want to put into the background picture and then use the move tool to drag it into the background picture. If the added picture is not the right size, hit ctrl+t or go to edit, free transform and you can resize it. To make sure it keeps it's shape make sure you hold down the shift key while dragging the corner to resize it.