As previously shown, a spray paint can contains the paint, the solvent, air and the little glass marble which lies at the bottom.
Shake it very well to mix up all the components.
Hold the can about a palm's width away from the wall when spraying--but this distance will vary depending on the effect you wish to create.
A tip: don't stop moving while spraying, or else the paint might drip. Instead, keep the can constantly moving while you paint.
The movement of your arm should come from the shoulders, from top to bottom, and not from the wrist.
Your movements should be large, like this.
Use the spray exactly as if you were drawing a picture, making light lines with constant, even motions.
Practice drawing vertical and horizontal lines to find the right distance between your hand and wall.
Try drawing circles and squares as well.
You don't need to go over the same lines multiple times since this will spoil the painting and make the surface uneven.
Practice until you're comfortable using only a single layer of paint.
Second tip: to better control the shape of your lines, try positioning the nozzle perpendicular to the wall.