What you want first of all is a good graphics program. I've already suggested Photoshop, but less expensive programs like Paint Shop Pro are also very good ([Link](Mouse over link to see full location)). You could do it in programs like Microsoft Paint, but you'll end up with unnatural-looking results.
Once you've got a good graphics program, learn how the program works -- programs like PS are very complex with a steep learning curve, so it's worth getting the hang of it. Since you want to copy your head onto a cartoon image, you want to be proficient with the various Select tools and Quick Mask in particular (PS-specific). Once you've got the hang of these, it's very easy to copy your head onto another picture, but for more details, have a look at tutes such as this, applicable to many situations: [Link](Mouse over link to see full location) [ viviena's advice column | Ask viviena A Question ]
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