Your options for learning how to build a frame are wide open. You can piece together an education from books, the many random videos on YouTube, and bribing or hiring your local builder. You can also spring for the tuition at a full-on frame building school. Some people combine some of these by learning general welding or brazing at a local community college and then fine tuning that with bike specific information from various sources.
If you know of anyone else doing a solid, reputable job of teaching frame building, please let us know!
The following are a few of the more reputable sources for framebuilding knowledge out there:
Dave Bohm - Arizona
BREW Bikes - North Carolina
Metal Guru (Carl from Vicious Cycles) - New York
Schon Studio - Squamish, British Columbia
Yamaguchi Bicycles - Colorado