Welcome to SR388! You've been sent here to exterminate the last Metroids before chaos begins. You'll arrive with your ship landing. If you're injured, go inside it. You'll reheal. Currently, you have 30 missiles, the Maru Mari (morph ball) and the power and long beams. Once you start, head right, defeating enemies as you go.
Soon, you'll come across a gap. Jump in the hole. You'll encounter some breakable blocks. Jump with and at the same time press down on the and to shoot down. This is tricky, but you'll eventually get it. After repeating various times, you'll meet a one block tunnel. Hold and release to morph into a ball. Roll through, and you'll have to destroy blocks again, but going up.
You will then meet a hallway to the right. Go through it, avoiding or killing enemies. You'll then be once again going down, breaking blocks until you reach some floor. This is a bell shaped rock, with space in the bottom for rolling through with the maru mari. Head right to see a save station. Stand on it and press when prompted on the lower part of your screen. That's all for the right side, for there is lava back there.
Roll left. You'll pass a few screens, and descend a cavern. Once you reach the bottom, continue left. After a screen, you'll face a Alpha Metroid; a more mature type of Metroid than you saw in the first game. Shoot them with missiles (press to switch.
If you're starting to get low on missiles (it's so hard to aim!) you can head to a secret alcove in the far right in the ceiling. Be sure to refill.
Now that you have defeated that Metroid, you'll hear and see a rumble, signifying a earthquake. It ends as soon as it starts. That means you've defeated all the Metroid in the area, and that the earthquake caused the lava recede deper. We can now enter the second area of the game, Phase 2. Head back to the bell rock, save, and continue going right.