Some general things you should know:
-Zair, super and homing missiles, and all of her aerials have no or very little landing lag. This makes platform landing/dropping your best friend for spam. Homing missiles are, for the most part, better than supers, since they come out much faster.
-Samus has an excellent off-stage game but overall has very low-knockback attacks, so you're going to get the earliest kills by gimping. Dair is the only move you should expect to frequently kill with below 120%. But her best kill moves in general are bair, dtilt, the Charge Beam, and an air-bound-target utilt, as well as the harder to land fsmash.
-Ftilt, dtilt, dsmash, and Screw Attack are really good out of shield and will replace her slow shield grab most of the time.
-The grab jab (AKA the Chozo Chop) is great for negating move decay.
-Screw Attack is amazing, especially on stages with high platforms like Battlefield in which Samus's free fall will end immediately by landing.
-Zair can be pulled off as a ledge hop attack easily in almost every stage by air dodging as you jump onto the stage and then zairing once you've passed the ledge (this doesn't work in stages with one-way pass-through platforms like Delfino Plaza and Halberd).