The frame size question can be quite interesting with tons of answers.
Start by finding your "cycling inseem" or "pubic bone height." Here is one place that explains how.
http://www.rivbike.com/article/bike_..._to_measure_it
Read, search and best of all if you have a bike shop in your area, see what they say.
Just guessing, in a mountain bike you will be needing something at least 21" in frame size.