Welders

Most importantly - learn from an experienced welder. It's very easy to put down a few beads that look good, then find out they'll break under any stress at all. I spent years as a welding supervisor at Newport News Shipbuilding so I've seen it all. Plenty of newbies like the look of their welds but there's very little penetration into the base material. And you definitely don't want a failure in a vehicle you're driving.
Also, do not reverse the polarity on a MIG unit unless you're switching from MIG to flux core. Makes it a bit easier to MIG overhead and on thin pieces but again, you lose penetration. One more thing - be sure you don't burn out your electronic ignition module welding on the vehicle. You may have to remove it before starting.