yay, mac!!! very easy - you open the picture, and should happen in Preview.app, then you just select the region you want to crop (press mouse button, drag, release button, then readjust if you want by dragging the borders of the frame) and cmd+k does it (cmd+s to save).
To resize Tools->Adjust Size (you can select resolution 72dpi since that's what the screen has, and I since the images in the classifieds get resized to 700x700 may as well pic the larger size to be 700 and get a better picture due to osx's algorithm being better than the one build into the classifieds software).