To reach the Varia Suit, you'll need to navigate through the lower, lava-filled sections of Kraid's Lair. From the main entrance to Kraid's Lair, generally head downwards and to the right. You'll encounter rooms with lava pools and enemies that spit fire. Be careful with your jumps and use the Morph Ball to navigate narrow passages. You'll eventually reach a large, vertical shaft with several platforms. Descend this shaft, carefully avoiding enemies.
At the bottom of this shaft, head left. You'll find a room with a large, glowing orb. This is the Varia Suit. Upon collecting it, Samus's suit will change color, and you'll immediately notice that the constant damage from the heat in Kraid's Lair ceases. Furthermore, the Varia Suit halves all damage taken from enemies, making it an incredibly valuable upgrade. With the Varia Suit, you are now ready to confront Kraid.