Zebes is composed of five major areas; the rocky Brinstar, the fiery Norfair, the two mini-boss hideouts for Kraid and Ridley, and Tourian which is home to the Metroid population and Mother Brain. In the game of Metroid, you are free to tackle any area in any order you want. The only limitation is that many of the more advanced areas require particular Power-Up items or weapons in order to access them. There is no hard and fast rule that says you must complete Kraid's Hideout before you enter Norfair, or even before you fight against Ridley. The approach you take is entirely up to you.
Brinstar occupied most of the top of the map. From Brinstar, you can access Norfair, Hideout I, and Tourian. Hideout I is directly beneath the blue (left) portion of Brinstar. Norfair is located below the golden (right) portion of Brinstar. From Norfair, you can return to Brinstar, or delve deeper into Hideout II, which is located beneath Norfair. Tourian is the gray area located in the upper left corner of the map, and cannot be access until Kraid and Ridley are defeated.
You are not bound to the order presented in this walkthrough; it is simply an example of one way to complete the game. Since time is a factor in determining what kind of ending you receive, you may favor an alternate method that reduces the amount of time that Samus spends traversing across the planet. Or you may choose to power up as strong as you possibly can before facing either mini-boss. The only hard and fast rule is that Ridley and Kraid must both be defeated before you are granted access to Tourian.