Alpha Metroids are the first Metroid mutations Samus encounters. They are the weakest form of Metroid by far and will fly forward quickly, but in short bursts. Because they are quite aggressive and appear in a variety of environments, they can be difficult for first-time players. Whenever an Alpha Metroid is hit with a Missile, it will fly back at an angle relative to the shot, so aim carefully. Since Samus can only fire one Missile at a time, the closer she is to the Alpha Metroid, the faster she can shoot. Since an Alpha is destroyed with only five Missiles, if you get close enough, you can fire all five in a fraction of a second.
You can cheap shot some Alphas (ones that aren't in the process of molting) by firing a Missile from a distance before the Alpha has registered your presence. The Missile will hit the Alpha, and when the Alpha begins to attack, your shot will immediately register as a hit. You cannot hit the Alpha more than once in this fashion.