Water can be your enemy. When installing tile in a shower/wet area it is CRUCIAL to have the proper substrate and waterproofing membrane before you begin. There are a few different methods of waterproofing and everyone has their favorites, but for us we like to use a product from Custom Building called RedGard.
RedGard can be installed with a roller, paint brush, trowel or air sprayer. Typically we use a small 2" brush and paint roller to apply it. Make sure that any gaps you have between your backerboard are sealed using drywall tape or mesh, this gives the RedGard something to stick to in larger seams. RedGard is more the consistency of pudding than paint so don't be afraid to put a heavy coat on your roller. Once your area is prepped it is time to start applying the RedGard. It is hot pink in color and when it dries it turns a dark red. It will dry with a rubbery texture so try not to leave any clumps anywhere as this will lead to unevenness when you are ready to apply your tile. We also like to mark about 1" INSIDE of where we want our tile to end. This allows us to go close enough to the edge but not out to far. If this gets on your painted wall you will have to paint over it, so we try and save ourselves the hassle and mark our area first. Make sure when you are applying that you get good coverage in any screw holes you may have, roll all your walls as close to your guide lines as possible. Once you have rolled everything, that is when we use the small paintbrush which works good for all your seams, don't be afraid to get a lot on the brush and go into your corners and seams. You want to make sure that the RedGard forms a tight seal over all of the gaps you have. Once your done, we typically let it dry overnight and then start our installation! Check out this video from custom to give you some more info: