Best way to start is to build some smaller kits for practice. For example, you should try building a few Velleman kits. They even have tube hi-fi amp kits:
A good place to start for a beginner who wants to build a tube guitar amp is the AX84 website. There are a couple of kits available for sale from AX84 users.
Once you know what you're doing, Mojo Musicla Supply sells everything you need to build tube guitar amps. Even cabinets, chassis and speakers. Costly, but very good quality.