Hello Liel! Welcome!
There are a couple of ways to crop you photo into a circle. One that comes to mind first is to do as you describ, select with the marquee the portion you want cropped, then hit
Ctrl D. This will give you a duplicate of the selection you have made, without cutting up your original (you may want other pieces from the original or want to even use it ole with the circular portion.)Just move them off from one another, and you'll see that you now have two- your orig and a circular one. Another way is to select as you did before, then hit
Ctrl C to cut and then Ctrl V to paste. Then just delete the original that now has a hole in the middle
.
Hope thi helps.
Laurie