Selection tools are one of the most powerful things in Photoshop because with them you can select a specific area of an image and then edit that selection independently from the rest of the image.
There are several selection tools available. The marquee tool is for a rectangular selection, there's a freehand tool which you can use to draw around part of an image and select it that way, and the magic wand tool selects areas of a chosen colour only.
As with everything in Photoshop there are many many options you can add to a selection like feathering which is the hardness of the edge of a selection. Foe example do you need a hard edge or a softer one that fades into the rest of the image?
Photo editing (http://www.photographycourses.biz/pho...) is a huge subject so we've made several videos and will make more..
Mike Browne