After a short descent, you'll find your first Metroid Larva. These are the weakest form of Metroid, but still dangerous if underestimated. They float around, attempting to latch onto Samus and drain her energy.
Strategy: Fire rapidly with your Arm Cannon. If it latches on, quickly switch to Missiles and fire to dislodge it. A few well-placed missiles will destroy it. After defeating it, the lava level will recede slightly, opening a new path. Continue exploring the newly accessible areas, looking for more Metroids and power-ups.