In addition to what Luke Pieta-peralta said the only way to get better it to practice, practice and more practice.
I remember in the 80's when the insurance institute mandated mig welding over brazing (torches), you should have seen how long it took us to get it right.
You can also get videos on how to mig weld
A2:To add what others have already mentioned, make sure you are using the correct or optimum shielding gas. If you are just short arcing solid wire, straight carbon dioxide will get you by, but a 75/25 mix with argon will provide a more stable arc.
I don't know what type of wire you are using, which process of wire feed (short arc, globular transfer, or one of the newer hybrid processes), using a multi process machine with syncho or square wave circitry, or even if your machine is a never inverter with synergenic settings.
For the most part, pay attention to what it sounds like when you are executing a weld that is to your liking. Every process sounds different. I about dropped the gun when I started my first bead on stainless. But when you know what it sounds like, learn to keep that sound consistent. Angle, stick out, and travel speed will cause audible changes.
I'm certified in Duel Shield and Self Shield wire. Weaving is an AWS acceptable practice. The puddle will act the exact same way as E7018 electrodes when welding vertical up. Horizontal welds benifit from a slight lateral occilation. It encougages both weld toes to tie in without excess convexity. But making "C" shapes is never acceptable. You're not stacking dimes like TIG welding aluminum. Each one of those spots is a perfect trap for slag, porosity, lack of fusion, or other discontinuities. Your goal should be a smooth bead, not too high in the center, with perfectly tied in toes-and no undercutting.
The other basic things would include not having a worn out contact tip, having the proper size and condition of gas cup, have a good ground, set your arc control all the way up if the welder has one, and be consistent-fluid in your movements. Any slight hand quiver, tremor, or cough is going to be visible in the weld.