I'm a horse mom newb myself, so hopefully some people with a better grasp on horse nutrition will chime in here. As far as finding out what weight your horse is at now, get yourself a horse weight tape at your local tack store, and even a plain old tape measurer will do. Horse Weight: Estimate It Easily
This site is a really great resource for understanding horse nutrition: Horse Feeding in THREE Easy Steps!
Beep pulp and chaff have been suggested to me as ways to help a horse gain weight as well.