What Andy said + I recommend making a joint out of 1" and 1 1/4" OD tubing with .035" wall thickness to start. The advantage of using a quarter inch OD difference is that it is easier in the beginning to fillet braze around the "ears" of the miter if they aren't so close in size to the tube it is butted up against. Those diameters and wall thickness are close to what you will be eventually making.
Don't expect your LWS to have either the materials or knowledge to help you achieve any framebuilding goals. If you buy anything other than C-04 brazing rod made by the Gasflux company with their Type B flux and sold by Henry James Bicycles or rod and flux from Wade Barcosi at Cycle Design, then you are getting off on the wrong foot.
As someone who has taught framebuilding for years I have pretty strong opinions on how one can best learn. I know what the learning curve is to be able to make a decent frame. Knowing how to teach a skill is different than knowing the skill itself. It actually can be discouraging to just watch a master do a fillet brazed joint. It will look easy and you'll think you can copy the same and then you try it and its all a mess. It begins with the right equipment and materials. During the demonstration there is a lot happening that the mind can't take in all at once or remember everything. How close, how fast, what angle the flame is, how the rod is held, etc, etc, etc. That is why before the demonstration there is a lot of explanation and review of principals. A good teacher knows all the beginner tendencies and how to correct for them before they are made. Let me give you just one example of many. It is a beginner tendency to hold the brazing rod down by one's side (or some distance away) when heating up a joint. When the heat indicators say brass can be added, now it takes them some time to get it in position. In the mean time the flame isn't being paid attention to and bad things happen. It is essential to have the brazing rod be really close to its entry point and be held at a certain angle. Otherwise too much attention is paid to the rod holding hand motion and not enough to the flame motion and things start to go south fast.
What method one chooses to learn how to braze partly depends on how good one wants to be. If one enjoys messing around for some time on their own with limited resources, then a DIY plan is okay if one doesn't care much about the quality of their first results. If one wants to be really good at it and have a learning curve that doesn't stretch forever, than they need to be taught by an expert that knows both the subject matter and teaching methods for different kind of learners. It really is that simple. I teach some short couple of day classes because I realize some will never have the time or money for a full 3 week class. When I wanted to learn I had to go to live in Europe so I'm probably not overly sympathetic that local training be a requirement.
Most average students can do a decent fillet brazed joint without much if any pin holes when they follow a training sequence that includes knowing what equipment and materials work best, have both general brazing instruction and are taught specific motions for each type of joint and are given a demonstration that illustrates all of the above.