How to make a realistic wood texture from scratch in Photoshop. (don't be scared, it easy)This is just a place to start.  You can, of course, just change some of the colors or settings and make many different colors and types of wood.

Start with a new document whatever size you would like. For this tutorial, I'm using a 256 square. If you are not sure what to use, go bigger.  It's much better to crop or scale down you final image, but scaling up will degrade the quality.

Next pick tow shade of the same color, one light and one dark. In the example, I used #6A5C3C and #AA9471, two shades of brown. Go to Filter>Render>Clouds. You want something that is kinda even with some lighter and some darker places. If you don't like your original result, hit [CTRL]-F to repeat the filter until you like the result. This will be the base for your wood.

Make a new layer above the others. Fill it entire layer with white. Go to Filter>Noise>Add Noise and use the settings monochromatic, Gaussian and about 40-50%. Lower the layer's opacity to about 40% and make its blending mode Overlay.

Next you want to make the noise look more grain-like by going to Filter>Blur>Motion Blur. For setting you should use 90 degrees and about 15 for the distance. Now you should have this:

The result is very nice basic wood texture. You can add an adjustment layer on top of the rest to alter the color and create several wood textures with just one file.  Just use Layer>New Adjustment Layer>Hue/Saturation and Layer>New Adjustment Layer>Brightness/Contrast then play with the settings of the two to get whatever look you want, from realistic to bizzare.  But you don't have to stop there.

To add a darker wood grain make another layer above your adjustment layers.
Use the brush tool and draw a few lines across this layer with black.  For the example below, I used a size 13 brush and 70 hardness.

Here is the tricky, but fun part. Use Filter>Distort>Wave and keep adjusting the settings untilyou get something you like, probably somethng smooth yet still pretty wavy. You can keep hitting randomize until you get something you like. Here are the settings I used and the result:

Although it doesn't look too bad, it still can use a bit of work to make it look better. Under Filter, the Liquefy tool is good for adjusting and ading other details. There are also a number of other tools you can use too, like the blur filters and the smudge tool. Remember that natural textures are not perfect and yours will look more realistic if its not either so just be craetive and see where it goes.

Set the layer blending to Overlay and lower the opacity until it gets something you like.  You can also adjust the color to create other effects.  There is no end to the variations you can make.