Hi SleepyGirl557,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choice … and know how and why to avoid the worst ones.
There is no specific material that is either better or worse for backs. The goal of a mattress is to find a design (regardless of the materials or components inside it) that keeps you in good alignment in all your sleeping positions. The type of mattress and the materials or components inside it are a matter of personal preference.
Post #2 here also has more information about latex vs memory foam.
Phoenix