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.
The following are a few of the more reputable sources for framebuilding knowledge out there:
The Bicycle Academy - England
BREW Bikes - North Carolina
Metal Guru (Carl from Vicious Cycles) - New York
Yamaguchi Bicycles - Colorado
Dave Bohm - Arizona
Doug Fattic (dougfattic