These tutorials shows how to "cartoonize a photo" using Photoshop. If you are new to Photoshop, please read this Adobe Photoshop CS3 Tutorial for Beginners.
For this example, a car photo is taken and the below steps are performed in order to convert the photo to

click to enlarge
a cartoon.
Tutorial 1
1) Open the photo in Adobe Photoshop. (The nature of the photo does not matter. It could be sports, beaches, people...anything. I have just taken a car as an example)
2) Apply a Gaussian Blur
to the photo by going to Filters --> Blur --> Gaussian Blur. Although you should get a cartoon effect from a blur radius greater than 3.0, I tend to use anything between 5.0 to 10.0 depending upon the image.

click to enlarge
3) Go to Edit --> Fade Gaussian Blur
. Choose Darken. Click on OK.
This is the end of the "convert photo to cartoon" tutorial in Photoshop.
Note: You can try repeating the series of steps mentioned above and using a different effect from "Darken" to end up with other interesting results.