Most people agree that Adobe Photoshop is the all around best graphics editor. It is an investment of several hundred dollars. They do offer a free trial so you can check it out before you buy. I currently own it and enjoy it very much. I have found a number of fun and educational tutorial projects on DesignFestival.
Starting out, I was not prepared to make such an investment. Two free programs that I found helpful are Paint.net and GIMP. Both have active communities supporting them. Paint.net is simpler reminiscent of MSPaint. GIMP is in many ways similar to Photoshop. GIMP is more complicated but also more robust. Either one will let you get the job done until you are ready to take the plunge and purchase photoshop.