If you can, take an intro course at your local community college. The intro
course at the one closest to me covers oxyacetylene (oxyfuel), stick, MIG
and TIG. You won't get good at any of these in an intro course, but you
will probably learn enough to decide what will make the most sense for you.

It is possible for some folks to learn from books. And I mean books, not a
book. But you will learn much faster with a knowledgeable instructor. And,
not everything useful to know about welding will be in the books you use.

MIG is by far the fastest to learn. It can be used for sheet metal through
reasonably thick material. I have seen people do groove welds on 1" plate
with MIG that passed a bend test. But be aware that when operated in
short-circuit mode, which is what you in all probability will be doing, it
is very easy to create something that looks like a beautiful weld but is
nothing more than weld metal melted on top of unmelted base metal. In
other words, no fusion. Because of this, it isn't used much for structural
applications. Stick or flux core is more common for these.

IMHO TIG will probably take the longest to master. And oxyfuel will be a
close second.

Stick is somewhere in between. With it, sheet metal (say about 20 ga. and
up with the right electrode, like 6013) through very thick material can be
welded.

As far as I am concerned, oxyfuel welding is obsolete. I am not aware of
any significant commercial use. Some homebuilt aircraft builders swear by
it for welding chromemoly tubular airframes. But TIG is better. The only
advantage I can think for for oxyfuel is it will work when you have
absolutely no source of electrical power.

A cheap AC "buzz box" stick welder capable of handling 3/32" electrodes can
be purchased new on sale for around $100 and represents the cheapest way to
get into welding. AC/DC units capable of handling 1/8" electrodes can be
had not a bit more. Your only addition expenses besides the cost of
protective gear you will need with any of the processes is the cost of the
electrodes and electricity.