How do I edit photos like this?

Photoshop, but if you want the free thing which does almost anything download GIMP. The picture you posted is two pictures merged together. The way you would do this is by first taking your picture of yourself that you want, then finding a picture of the pretty stars, and in the program you would "Add Layer" and the stars will be the second layer, which laysover the picture of you, and then all you do is lower the opacity so it becomes translucent :)