The first step to becoming a modern master is to choose a photo that might lend itself to an impressionist treatment. Landscapes and nature shots tend to work well, as do subjects with strong graphic shapes, repeating patterns and a decent amount of detail. When all that detail gets filtered, it's the stuff that provides the painterly feel.
As I always suggest, duplicate the image onto a new layer. In this case though, it's for more than just safety reasons. The opacity of the new (and eventually filtered) layer will eventually be reduced to reveal original image detail at the end of the process.
On the duplicated layer, choose Stylize>Glowing Edges from the Filter menu. When the dialog box opens, adjust the Edge Width, Brightness and Smoothness. The names are self-explanatory; adjust the sliders until the effect is strong but doesn't completely obliterate the general details of the original. Thick edge lines are great and they contribute primarily to the success of the Van Gogh look. However, with too much thickness all the detail will disappear.
Next, adjust the layer blending mode to Screen and watch the lightest details of the newly impressionist layer remain. It also allows some of the detail from the original image layer to show through. To see more of that layer, thereby reducing the strength of the Find Edges effect on the top layer, reduce the top layer's opacity until you're happy with the results.
I like to take this process one step further so it looks more like an Impressionist painting and less like a photograph. Because so much of the original image detail is showing through the top layer, run a Gaussian blur on the original layer just to take the edge off some of the details and produce a more painterly effect. When you're happy with the look, merge the layers down and save your new file. Print it out on canvas and hang it on the wall-right next to your real Van Gogh.