OK, I've been MIG welding a long time. I can gas weld, arc weld and MIG weld. I have been teaching welding in classes for some time. I however, do not have my certification. I just want to clarify. Now on to the situation at hand.
MIG welding is the easist welding to learn, gas welding being the most difficult IMO. You do need to practice, and not on your frame. You could get an old frame from the junk yard or such, but you need some type of tube. I like to use .025 wire on about everything. Even the pocket MIGs can weld up to 3/16" and with beveling, you can go thicker. You will have no problem MIG welding tube with your welder. It is all about the heat settings and the wire feed. Heat settings aren't too bad to figure out. Your welder prpbably have 4 or 6 heat settings.The second setting is probably about right for you. The wire feed is the trickiest and makes the most difference. So you will have to experiment until the welding sounds like cooking bacon. I nice even sizzle. Come back and ask more after you have tried.