You can do pretty much anything you want to a raster (pixel-based, like a JPEG) image with Photoshop. The only thing you can't do is enlarge a picture that is really small. It will look bad, no matter what you do.
If you want to change someone's eye color in a picture, you can use the lasso tool (top of tool panel, looks like a little loop thing) to select one eye, then hold down shift while you select the other eye. That tells Photoshop you want to select another area, if you don't hold down shift, it will deselect the eye you already selected. Alternately, you can click "add to selection" on the toolbar After you've selected both eyes go to Image>Adjustments>hue/saturation and play with the sliders until you've changed the eye color to something you like.
Download a free basic Photoshop guide here: http://www.adobe.com/cfusion/marketplace/index.cfm?event=marketplace.offering&marketplaceid=2&offeringid=12373
I really recommend that you get the Photoshop Classroom in a Book. It's an expensive book, but you'll learn to use the software to the extent it's intended to be used.