i just can't see learning to weld online, you can learn anything they would teach from a beginners book! practice is what it takes...i take for granted that i'm a 3rd generation welder...i grew up around it, and by 12 was fabbing up my own gocarts and at 16 built my first trailer to haul my lawn mower that i financedand paid for by mowing in 1 summer! in my engineering classes i did my semester of welding activities in a week 1/2 but i had worked 8 yrs pipelineing, food grade shed. 10 ss to 6 inch thick carbon steel....i think the most nervous i've ever been was when i was watching a ??? ton dmag(only 5 in usa largest in states at time...1995-6?) and a 60 ton sum'n together lifting a 880,000 lb extractor that i had welded the lifting lugs on! vivid 15 min memory.. :-),
for your stated use i would go mig w/gas shield then you don't have to worry about learn what liquid flux looks like next to your puddle, you only puddle the steel and w/gas shield thats all you see and practice...thin stuff is harder to weld than heavy weight stuff, if you want the ultimate on your cage learn the tig and its easier to weld the thin stuff with the tig imo....but is tough to learn on, but once you do, its the strongest easiest to weld w/imo....better heat control and add filler as needed, so you can sit there and arc w/o puttin metal down that forces you to move forward, some thin light weight stuff, like when i shortened a truck bed, i didn't use hardly any filler, just puddled the sheet metal together
i have an old forney welder, that you move the cable to change amps, bottom of it is batterycharger:-), and my tig w/ diff % tungsten "needles"(the proper name eludes me) and a bottle of gold or tri mix gas and weld everything w/ 1 of the 2