If I am not using a template (where the shapes are ready and you can clip your photo to it) -
then I will use the ellipse tool and draw out a circle, have the photo layer above the circle and clip it to the circle by using f.ex Ctrl+ALt (Windows - I don't know Mac commands). When you are in the photo layer you can easily change the position of the photo, change it's size etc.
I am really BAD at explaining anything really, so I hope Cassie will get to my rescue and explain it a little better!
Actually, if you are new to digiscrapping, you should just go ahead and buy
Cassie's tutorials - you will learn LOADS and LOADS from them....