You make/edit textures in image editing programs, common programs are Adobe's <a href="http://www.adobe.com/products/photoshop/main.html">Photoshop</a> and Jasc's <a href="http://www.jasc.com/products/paintshoppro/">Paint Shop Pro</a>. Create your textures, and save them making sure the dimensions are multiples of 16. Once you have the image files, put them into a WAD file using <a href="http://www.telefragged.com/wally/">Wally</a>. You can then edit them to your liking there, and save and use the WAD file in your maps.