Hi mrhoades !
Welcome to you 
All I can say about this thumb story is that when you learn to play open chord (the first we usualy learn like A, E, Am, C, G and so on) try to learn them with the thum behind the neck, it’s a good habit if you never played before.
The thumb is used to mute the low E string when you play C, A, D, etc.. all chords who do not require this E string to be played.
But you just have it to move a bit to just touch then strings. At first try to find a natural position, wrist down, fingers with a “cup shape” and thumb behind (up but not too much) the neck .
You can feel little pain at start in your wrist but it is normal. If you feel big pain, stop.
Hop it helps and sorry for my loosy english.