Hey there, if you havent build a tank before maybe 100 + gal
is a little ambicious, but if you are set upon it here is what I can help you with.
In regards to glass thickness you can use the following table:
But before that you must set the dimensions yourself to see which thickness they requiere. Throw some numbers around depending if you want it a bit taller or longer and use one of many aquarium tank volume calculators which you can Google. Hope this helps, maybe others can throw some advice as well, take care.