The simplest answer to "how should I learn to weld" is... buy a welder and start gluing metal together with fire. That's how I did it. Pick a couple non-critical projects for your first ones... a coat rack for the house, then a bumper, maybe a small spare tire bracket. Hold off on projects like shortening your trucks frame for a while... for obvious reasons. Also, start on things thicker than 1/8". Sheetmetal is a lot trickier - especially with wire thicker than the thinnest stuff (.023) and the really thin sheetmetal used on imports and newer cars.
Wire-feed welding is dead simple to learn. In about 2 hours of total bead-laying time you'll be somewhat proficient. It didn't take me long to be somewhat competent... It took a good while longer to be 'good'.
Another thing: If you MUST buy a 110V welder, make it a decent one like the Hobart Hander 135 or similar Lincoln or Miller products. The cheapy welders like a Century or no-name will frequently be rated at 80 amps or less. Anything smaller than 100amps is STRUGGLING to penetrate 1/8" thick steel... and virtually any bracket you'd want to weld to your frame, or axle, or pretty much anything of substance on a vehicle is thicker than that.
Also, tiny welders on thick material are frustrating. My buddy still has that century in the above picture. I have to really, really be a team player with it to get a 1/8" bracket to stick to a framerail or similar. He uses a stick welder on anything thicker than sheetmetal... he bought it only for that.
My first unit was a Millermatic 175, which is what I still use. Many people shy away from it because it's 220volt-powered, and it's about a 600 dollar machine. It's the most expensive tool I own, but I'd sell my torch, engine hoist, and every hand & air tool I own before I got rid of it. I built a 48" wide plow with 1/4" thick teeth to drag behind my father in laws ATV last night. Welding 1/4" plate to a 2" thick solid steel bar is pushing the limits of that welder... but it got the job done with some really pretty, strong welds.
The first thing I did when my Miller showed up from cyberweld.com (good prices and great service) was grind all the crappy 110V welds off my rear bumper on my 4x4 and glue everything back together with the Miller. I've since had both my truck and a fullsize chevy hanging off the rear tow point while I winched him up a 40-degree hill. No problems.
That bumper was the first project I did with my Miller. I used the baby-welder to shorten the bed 12 inches. They work great on sheetmetal, although the big boys can be adjusted down to work slightly better.
