I searched the internet for practical information on MIG welders and welding and found nothing for the beginner. There were adds and odd references but little basic information. Even the user manuals I've seen (or heard about) assume you know the basics. So here is what I had to learn while buying a MIG and learning to weld.
The primary reason I wanted a MIG was to weld 3mm walled aluminium tubes into a rail system for a chainsaw mill I designed. I can do basic stick welding but I'd never even seen a MIG up close so it was all new stuff.
Around 25 years ago my brother (RIP) who was a professional welder told me about the MIG welding he did. He welded petrol tankers and serious stuff like that. He told me M.I.G. stood for "Metal - Inert - Gas", this may have been the case then but modern gas mixtures make this acronym obsolete, I'm told it now stands for "Metal In Gas" which is not accurate either because of "gasless" MIG welding. Anyhow the jist is that stick (arc) welders use a coating around the stick/electrode which melts,vaporizes or burns to provide a barrier/shield preventing air reacting with the molten metal. A MIG uses a gas or gas mixture to displace the air around the arc - also to preventing air reacting with the molten metal. The "electrode" is still melted with and electric arc but the electrode is a special wire which is mechanically fed into the arc. The feed rate is adjusted depending on the thickness of metal being welded - the voltage/current is also adjusted.
I could not really justify owning a MIG all by myself, they are much more expensive than stick welders and usually the gas bottle is rented so there is an ongoing expense whether you use it or not. A friend, Phil agreed to buy one jointly with me. I read specs on several MIGs via the web and figured I'd want one that could deliver at least 150 amps - I think I got that about right. I meantioned our interest in buying a second hand MIG to a farmer who belongs to my gliding club and he offered to sell his - a WIA150 - ie 150 amps. He wanted a bigger one, this welder was fairly new and the price AU$900. We took a look at it and I did my first MIG weld. It was pretty much as I expected - it feels a little like squirting liquid metal onto the job. I welded some mild steel bar and some sheet steel and got the hang of it fairly quickly. The owner had set up the MIG and I was spoiled by his electronic welding mask. Unlike stick welding it's recommended you weld away from yourself to improve the gas shielding. The other odd thing is - this welder controls the welding voltage not current and it does this with 3 switches (ie 8 settings), I would have preferred a knob or even a rotary switch but I'll get used to it. The other annoyance was this welder didn't take "mini-spools" which I'd been told were a standard and I'd already bought a 500 gram mini-spool of aluminium wire. We pondered for a week or two and then went to buy it. We were hoping to borrow the existing gas bottle but then we were informed that this gas was for steel and unsuitable for Aluminium. This made us ponder a few minutes more. We didn't want multiple gas bottles. We knew about "gasless" wire so we figure we'd get the gas for Al and use gasless wire for steel. We re-spooled my Al wire onto a bigger spool, paid our money and took the MIG to Phil's place. I set up the MIG and chatted to some passing missionaries while Phil got some gas. The D-sized bottle of Argon cost AU$55 - that's the price of the gas, the bottle rental is extra. We had no info regarding voltage settings or feed rate. I practice for hours on some scrap Al and mostly made a mess and broke lots of wire. Eventually Phil asks his neighbor (a pro welder) to take a look. He declared we owned a nice little MIG and the problem was we had the wrong tip size for the wire - DOH! The previous owner use one tip for everything but steel must be easier to weld. We had .8mm wire and a 1.2mm tip. Later Phil read the manual more carefully and we also had the wrong sized drive roller. The following week Phil got a range of tips @ $2.50 each but could not get a new drive roller. The local agent told Phil our welder could not do Al - this is a load of rot. They latter changed the story to say the didn't recommend it for Al because the max current was to low - for 3 mm thickness this is not the case.

I returned to Phil's the following weekend and produced acceptable welds within a short time, not pretty but they are strong and did the job. This was joining 25mm square tubing to 50*25 tubing - both 3mm wall thickness. The feed rate was set to the maximum setting of 10 - this could be 10 meters/minute but I'm not sure. The voltage was about 3/4 full voltage. We still had the .9mm feedroller installed driving .8mm wire..

My next batch of welding didn't go well though. This is almost certainly because the respooled wire had subtle bends in it which didn't pass through the tip smoothly and caused more wire breaks and jam ups. Using an over-sized tip reduced this problem but this also reduces the quality of the weld.

About gas.
Ideally different gas mixtures are used to weld different metals. Mild steel uses argon mixed with %25 carbon-dioxide (CO2), aluminium uses pure argon, stainless uses argon with one or two percent oxygen (O2). However we learned that even some pro welding shops use argon for everything. The CO2 keeps the carbon content in steel weld up and the O2 keeps the stainless steel soot free. We can live with low carbon steel and dirty stainless steel (stained steel). For our hobby/business welding we'll use argon only.

About welding wire.
The wire spool lives inside the body of the MIG unit and is driven through the centre of the "cable" connected to the "gun" part of the MIG. This cable also feeds welding current to the tip, supplies gas and contains signal wires to the switch on the "trigger". The drive motor is variable in speed so the wire feed rate is adjustable. Unlike stick welding where you choose the thickness of the stick/electrode according to the thickness of metal being welded - with a MIG you change the feed rate instead. Because the wire is thin it's possible to weld thin metal, the wire is also unlikely to stick when cold like stick/electrodes do. If you loose the arc the wire will melt rather than stick. Our MIG can take wire from 0.6mm to 1.2mm. Our steel wire has a thin copper coating to prevent corrosion. You can weld steel without using gas if you use a gasless wire. This wire contains a core of chemicals which provide the same function as the chemicals around a stick/electrode. Presumably you end up with slag covering the weld when using the gasless wire. The wire is more expensive but this is compensated by not needing to pay for gas.

About welding direction.
Both the manual and advice from experienced welders says you should be welding away from yourself when using gas shielded wire ie pushing the gun not pulling it. I guess this is so you're moving into the gas envelope as you go. However when you do this you're often stricking the arc on top of the molten metal bead and the motion is somewhat un-natural. It's a bit like using a caulking gun - running the bead away would make a mess. If you run the weld the other way you can easily keep the arc at the joint where you want it and it's basically much easier. You're also not passing near the hot welding as you go. So the last weld I did I broke the rules and went the other way - the result was the best looking MIG weld I've ever done. I don't have lab to test the weld but it's much neater looking and has the best penitration I've seen so far - YMMV.