After landing your ship in Crateria, proceed right, past the initial enemies, until you reach a long vertical shaft. Descend this shaft, being mindful of the Mellows on the walls. At the bottom, head left to find a Save Station. Save your game here. Continue right from the Save Station, past the Missile door you can't open yet. Drop down into the next area. In this new room, head left to find a small chamber. Inside, you'll discover the Morph Ball upgrade. This allows Samus to curl into a small ball by pressing 'Down' on the D-Pad, enabling access to tight spaces. This is a fundamental ability for exploration.
Once you have the Morph Ball, backtrack to the vertical shaft you descended earlier. On the right side of this shaft, near the bottom, you'll notice a small, horizontal tunnel. Enter Morph Ball form and roll through this tunnel. This path will lead you to a new section of Crateria and eventually to the entrance of Brinstar. practice using the Morph Ball; it's crucial for navigating many areas of Zebes.