How do you roll in Super Metroid?

Once you get the morph ball upgrade, press down twice on the d-pad (or keyboard).

To get the morph ball, start from your ship and go through the door on the left. Jump over the first big tunnel in the ground, and go down the second. If you can't fit, wrong tunnel. Go as far down as you can; there should be a blue door on the ground. Go through it. Hit the bottom of the shaft, and go through the door on the right. After another door, you'll find an elevator to Brinstar. Enter the door on the left, and you'll find the morph ball.