Alpha Metroids are one of the earliest Stages in the Metroid Evolution Chain, and because of this, they are pretty simple to defeat. By merely tracking the repetitive motion and actions of an Alpha Metroid, a player can destroy the creature with ease. Alpha Metroids have the tendency of floating to the top of a room before slowly backing up and then lunging at Samus.
Right as it tries to attack, nail it with one of Samus' Melee Counters. This will stun the Metroid long enough for Samus to get in a few shots. So, after you Counter the Metroid, quickly switch over to Missiles and launch a few its way. Missiles are one of the few weaknesses of Alpha Metroids, as well as Samus' Ice Beam. If you prefer Missiles, try going for the Metroid's weak point: its squishy body part. If you want to try out your Ice Beam on them, aim for the belly of the Metroid. This will Freeze it long enough to give Samus more opportunities to strike.