I’m having trouble getting my selection perfect using the marquee tools - any tips?

The transform selection option under the select menu is very helpful to adjust the selection shape. You can also click and drag inside the selection, with the marquee tool selected, to move the selection OR use your keyboard arrow keys for greater precision. With a circular selection, consider positioning your mouse in the center of the shape and using the shortcut ALT/OPTION and, if needed, SHIFT, to expand the selection outward from the center, proportionally, if needed.