How to put a texture on a specific side on the prim
You've rezzed a prim, now you want to texture it, right? But wait, you don't want to color all the sides of the prim in the same texture!
In the build toolbar you can see the option to 'Select Texture', select that option. Now click on the side you want to put the texture and you'll see it's highlighted in white. Now go to the 'Texture' tab and choose your texture and 'Select'.
Vwolla! You have textured one side of the prim!
You can also hold SHIFT and select a couple of sides, and also select the whole prim and then go to 'Select Texture' and uncheck a few sides.