Go to Filter> Distort> Lens Correction and in there, adjust the vignetting to add a vignette that you like. Usually with lomo photos, they have a very strong vignette.
Now select the Levels layer and then go to Layer> New Adjustment Layer> Brightness/Contrast. It should appear as the top layer.
In the Brightness/Contrast, checkmark use legacy if you're using Photoshop CS3. If you are not using Photoshop CS3, then you don't have to checkmark anything. Now reduce the contrast to -25. This simulates the low contrast of lomo cameras.
Here's the final results of this lomography Photoshop tutorial.
