What camera are you using? Programs like Aperture and Lightroom are designed to work with DSLR cameras that put out RAW/DNG files. If you are wanting to play around and are using a point and shoot - try Photoshop Elements. Does a lot of stuff that Photoshop does in the same manner. Pretty easy to use and a lot of help/books out there.

If you don't want to invest in anything you can always try GIMP
GIMP - Downloads
I have used it and if you have never used any photo manipulation program - this is a good place to start.
There is a more Mac like port called seashore
Seashore
I would recommend trying both those out as they are Free/Open Source so they don't cost you anything.