A quick & effective way is to put a layer on top of the layer with your photo, and fill that top layer with a transparent-to-white gradient.
1. Open the photo in photoshop, create a new layer (we'll call it "gradient layer")
2. Set your foreground color to white.
3. Select the gradient tool. In the tool options bar across the top, click on the dropdown menu of preset gradients. Pick "Foreground to transparent". Then, next to the dropdown menu, make sure you have "linear gradient" selected (the white-to-black icon).
4. Make sure you are working on your gradient layer. Position your gradient tool near whichever edge of the photo you want to fade and click and drag toward the other side (hold Shift to drag in a straight line). So if you want the bottom of the photo to fade into white, you would start from near the bottom edge and drag up. Everything beyond the point where you start would be completely white, and everything to the other side of the point where you stop would be completely visible.
5. The advantage of having the gradient on its own layer is that if you need to make small tweaks to where it starts and stops, you can just hit Ctrl-T to bring up free transform and scale the gradient layer to your liking.
If you need to fade more than one edge, you can add additional layers and repeat steps 2-4.
Once you're happy with it, do Layer-Flatten Image and save it as whatever output format you use.