Tiling a shower can seem more simple than it is. Too many people slap down some backerboard and lay tile, only to realize a few months later that they really screwed up. You'll definately have to lose the existing shower pan. If you're tiling a new one, I'd suggest doing your research. There is a lot to doing it right. One mistake can be catastrophic. Once you understand the process, doing it right should come easily.
The best option in my opinion is to use a product called Kerdi over a sloped mud bed or one of Schluter's pre-formed foam shower floors. The Kerdi is a waterproof membrane that is set in thinset mortar (very easy). You tile over the Kerdi. Check out youtube for some installation videos.