A seamless texture is nothing out of this world. In simple terms, a seamless texture is an image that can be placed side-by-side with itself without creating a noticeable seam between the two copies. They are not that difficult to create. The trick is making them less repetitive so that when you apply them on a big area, there isn't any noticeable pattern.
Note
If you don't want to lose time with this, Cgtextures.com is a good place to go because they provide tiled or seamless textures.
Again, we will have to stick with Photoshop to create a seamless texture, since we cannot cover here all of the image editor applications available.
Note
If you are not too concerned about getting perfect textures, you may want to have a look at the free seamless textures generator available at http://www.the-orange-box.com/.
Start by creating a square texture and keep in mind the image size. After that, select the Offset tool and, in both the Horizontal and Vertical text box, insert...