When learning to MIG weld, one of the biggest things you can do is to listen to the sounds your welder makes when welding; and also knowing what the error is when looking at a weld. Once you understand what to look and listen for, you will notice your welds improving dramatically.
A properly adjusted MIG welder should sound like it is frying up bacon when laying a bead. You want a nice sizzle with little "pops and spits". The final weld should be relatively flat and even throughout. Above you can see some examples of MIG welds. The first weld has our MIG 175 settings dialed in pretty good. Notice how the bead isn't protruding from the work surface too much, and the bead is fairly uniform in width. The backside of the work piece will show a nice outline of where the weld penetrated properly into the metal.
On the second bead we turned the wire speed up way too high. You can instantly tell the difference in sound and look of the bead. You can also feel it in the MIG gun when welding. The problem here is that the welding wire is coming out too fast for the surface you are welding on and the heat settings you are using. The wire is hitting the surface and not melting into the metal fully. You will feel the wire pushing back on the MIG gun because of this, and you will hear a lot of random popping from the welder. Lastly, you can see in the picture that as we moved along the welder just made individual "globs" but no puddle was formed. With this weld you won't see an outline of the weld on the backside of the metal as it hasn't penetrated into the metal. This is very bad, and is a weak, dangerous weld.
The third bead we did quite the opposite, we turned the wire speed far too low. When you do this you will notice a pronounced "hissing" when welding. This will sound like you have a gas leak. You will also notice the wire burning before it even gets to the surface and the inability to move the puddle. You can see in this example the puddle started, but as soon as you move a little bit, you lose the puddle and the welding wire isn't coming out fast enough to keep up and add to the puddle. Because of this you can see the arc was still present, but the puddle didn't start again until we stopped moving and let the puddle form. This is also a weak, unsafe weld, and should not be used.
The fourth example is a common mistake when you first begin welding. In this bead we left the shielding gas bottle turned off, and attempted to run a bead without the aid of shielding gas. When this happens, there will be an extreme amount of popping and hissing, as well as an excessive amount of sparks and slag flying from the work area. The shielding gas cleans the weld puddle and keeps contaminates from entering the weld and making it weak. Here you can see the weld is very porous and much darker in color. Keep this in mind when welding. If you see a weld like this, make sure you check that your gas bottle is turned on, the tank output pressure isn't too low, and the tank isn't out of gas. DO NOT attempt to use anything you have welded without shielding gas, all of those pores are imperfections that will make the weld weak.
Now go out in the shop, set up your welder, turn the music down, and listen to what your MIG has to say. It has a lot to tell you!