Welding is not inherently difficult but it does take knowledge, practice, a steady hand, and the correct equipment. Being able to diagnose setup problems, select consumables appropriate for the process and weld position, and understanding joint geometries is critical to your success.
The standard reference for welding basics is Lincoln's Welding Handbook. I would recommend it to anyone involved, or getting into welding.
There are numerous welding processes of varying complexity and cost but common ones are Shielded Metal Arc Welding (SMAW, aka Stick), Gas Metal Arc Welding GMAW, aka MIG), and Flux Cored Arc Welding (FCAW). You can buy hobby rigs at Home Depot and such that will perform any of these but they are limited in capacity. For the average Do-it-yourselfer a 200 to 250 amp unit such as a Miller Millermatic, Lincoln Power MIG, or ESAB MigMaster offers tremendous flexibility but they're not cheap.
We have SMAW, GMAW, and FCAW processes in our shop but most of my work is with GMAW or FCAW for materials measuring 5/16" thick or less. For applications such as an Airstream frame repair my machine/process of choice would be a Millermatic 252 running an E71T-X FCAW wire, or an ER70S-X GMAW wire.
I urge anyone to spend a fair amount of time practicing your skills before attempting any project welding. I would also add that most Vocational Welding Programs in this part of the country are more than capable and willing to do work of this type for the benefit of their students. Usually the cost is no more than the donation of materials to do the project.