You need to learn a bit more about shower pan construction. Kerdi might be better if this is your first time.

Here is a link that might be useful: Terry Love discussion

Redgard is fine, but read the application instructions to see if it's right for you.

I've been using Kerdi for well over 10 years, personally, I prefer it over Redgard for residential showers. Here's a Kerdi Shower Thread from the bathroom forum.

It's a fairly inclusive thread that should answer most of your questions.