Mancaves,
Learn by reading and doing? Sure.
www.millerwelds.com
including
http://www.millerwelds.com/resources/calculators/mig_solid_amperage_calculator.php
and look for the"Other Resources" links on the left, including
http://www.millerwelds.com/resources/basicMIG/index.htm
www.lincolnelectric.com
including
http://lincolnelectric.com/en-us/support/Pages/resources.aspx
and
http://lincolnelectric.com/en-us/support/process-and-theory/Pages/process-and-theory.aspx
www.esabna.com
especially
http://esabna.com/us/en/education/esab-university.cfm
and
http://www.esabna.com/EUWeb/MIG_handbook/592mig1_1.htm
Next, as to what you have and what you want to be able to do.
You have a DC CV (constant voltage) welding power supply used for wire-feed welding (GMAW aka MIG, or FCAW). By itself, it won't really enable you to weld.
To weld, you also need some sort of wire feeder. A wire-feeder with a gun/torch or a spoolgun.
http://www.millerwelds.com/products/wire_feeders/
Typical Miller wirefeeder choices for use with a CV power source would be a 22A/24A 'shop' feeder or a Suitcase 8RC (8 inch spools) or 12RC (8 or 12 inch spools) feeder for shop/field use. Since your welding power source that you have is CV, you do not 'need' the VS suitcase wire feeder voltage-sensing capabilities, although they do give you the capability to run up to 400 ft (round trip) of welding cable between the suitcase and the power supply compared to 150 ft round trip of the RC suitcase feeders.
You already have a spoolgun.
Spoolguns are typically used for wirefeed welding of aluminum, as the aluminum electrode/filler-wire is typically soft and not able to be fed through a long gun/torch cable (think pushing a rope uphill or pushing cooked spaghetti noodles uphill through a thin straw several feet long). The spoolgun shortens the distance the (soft, compared to steel filler wire/electrode) aluminum was has to be pushed from multiple feet (10, 15, or 25 feet often for 'common' guns/torches) down to maybe 12-15 inches.
Like most things in life, there ain't no free lunch. Meaning that there are always trade-offs. Spoolgun trade-offs include bigger and bulkier than just a gun/torch head, so tight access is reduced when using a spoolgun. Also, the spoolgun is heavier than just a torch head. And spoolguns just use the small 4 inch diameter spools of filler wire (typically 1 lb in aluminum wire and 2 lb in steel wire, solid or fluxcore). The small 4 inch spools are typically more expensive on a per pound basis than larger spools and not all wire types/sizes are available on all spool sizes.
Want to learn to 'MIG' weld steel? Easiest way I can think of, and costing less than getting an 'industrial' Miller wirefeeder (shop 22A/24A or Suitcase), would be just buying a 'small' GMAW aka 'MIG' machine. Typically, the 180-class of machines are quite capable (within their power limits). Moving up to the 250-class machines gives you more capability as they can usually take the larger 12 inch wire spools compared to the 180-class that can usually only go up to the 8 inch spools, and also usually have enough power to let you do spray-transfer as well as short-circuit transfer (read the above links for what that means if you don't already know). Again, no free lunch as they are typically bigger and heavier and more expensive than the 180-class machines.
Buy the small(er) MIG machine, learn to weld steel, and then resell it. Or keep it, as a handy smaller machine. Even a (decent) 120V MIG machine is usually quite nice for GMAW on sheet metal (auto panels). Rig it with some 0.023/0.025 solid wire and a tank of C25 and you are good to go. Note that welding on thin sheet metal is an exercise in not only ADDING heat to weld it but in also MANAGING heat to not warp it into a banana.
Regarding classes and school, instead of looking at the vo-tech schools, maybe check out some 'adult ed' classes. Those typically run evening/nights instead of days like vo-tech stuff does.
And as mentioned (several times), to MIG weld you need shielding gas. Different shielding gases may be needed based on what and how you are welding. For short-circuit transfer GMAW of plain steel, two typical gases used could be plain CO2 or C25 (75% argon and 25% CO2). For welding stainless steel with GMAW, different gases are used. For GMAW of aluminum, plain argon is used.
There are some FCAW wires that can be used (quite well) for welding on steel. Quick description of FCAW wire? Think of a stick electrode turned inside-out with the flux inside instead of outside. Some FCAW wires are made to be used all by themselves (FCAW-S for "self-shielding") and some are made to be used with gas shielding (FCAW-G). Note that the FCAW-G wires (and many FCAW-S ones as well) can't be run with the 'little' MIG machines (they don't produce the correct voltage output to run the wires). Your Regency 250 would not have that problem, as it has enough volts and amps to run most any wire type (but not all sizes, as it still tops out at 250 amps max).
FCAW wire typically burns 'hotter' than plain solid wire with gas. So in combination with also producing slag and typically more spatter than GMAW, FCAW is -usually- not the go-to method for welding thin steel (like auto body panels). There are also issues with the slag being on the 'back' side of a welded patch panel, that if you can't get to in order to grind it all clean can lead to accelerated corrosion/rusting from the back side of panels. As mentioned.
Like most things in life, it's all about trade-offs and choices. Your call as to what and how you want to rig/do things.
