Your biggest problem is the Mom hurdle you need to get over. The best way to do that is to think like a mother. So lets start...
#1. "My baby is playing with fire" Mom's are programmed forever for as long as they live to "protect". So you need to be the role model of safety. I suggest getting a pair of safety glasses to start.
#2. "Fire is dangerous" Women in general have a strange adversion to fire. Dont ask me why after two wives and numerous GF's they all had the fear of anything burning. Except candles... Women like to have a house. So you need to ensure your mother that your not going to burn her house down.
#3. "Propane explodes" See #1 & #2. I highly suggest you consider building something other than a propane forge. Google "Charcoal Forge" you should find some excellent information. Plus natural charcoal is easily found at wal mart, lowes, home depot.
#4. "He'll make a sword and kill somebody" If you've told your mom, or anybody else that would tell your mom, that your plans are to make a sword or knife or any other form of "weapon" You've blown it for now..period...end of story
I suggest you concentrate on your grades and sports. Nothing makes moms happier than good grades. It is a major factor in them judging responsiblity. Check out some books from the library and make sure she sees you reading them. Get some clay and play around make some animal faces and other art'sy stuff. By spring you'll be beating hot metal in the backyard.