The first order of business will be butt-welding two pieces of 1/8-inch mild steel together. With each side of the piece tack welded, this illustrates how to position the gun and in which direction we're going (notice my left hand helping steady the gun). There is no official direction you need to weld, you can go right to left, left to right, up, down, and so on. It just depends on the situation and making the weld any way you are comfortable with. | 
While this is an exaggerated "push" welding angle, it illustrates "pushing" the filler wire into the weld. Pushing helps fill the area about to be welded with shielding gas, but is also pushing heat into a cold area which isn't as good for deep weld penetration. Also notice the wire sticking out of the nozzle, this is called "stickout." Be sure to keep the amount of wire stickout from 1/4- to 3/8-inch while welding. | 
Whether you use a push or pull technique (aka drag) shown here, do so with about a 10 to 15-degree angle from holding the gun perpendicular to the work piece. Pulling will give deeper penetration than pushing along with a narrower weld bead. |
Here we have a sample of what happens when holding the gun at a 90-degree angle to the piece as well as pushing and pulling techniques. | 
When it comes to the actual speed you should weld at, it all depends on the material and its thickness as well as direction when it comes to vertical welds, since gravity comes into play with a molten weld. In any case, you don't want to go too slow and create too much heat and cause warping or burn through, and traveling too fast will not give proper penetration. Getting a feel for this will only come with practice, and if you can watch a good welder have at it (with a helmet on of course), you can also learn a lot. | 
Many people find it easier to learn to MIG weld by practicing a fillet weld (welding into a corner). A fillet weld's two pieces coming together to make a corner, helps give the weld a place to go, and is easier to see where you need to weld than butt-welding like we covered earlier. All the same gun control techniques still apply. |