Basic How to select MTB forks

So many forks and so many sizes to choose from.
Basically, you need to look for few precise things before knowing a forks may suit your bike.
First there the length. The frame you have has been precisely designed to fit a certain forks length .
This length is important on how the bike handle.
To evaluate the fork length, it is measured from the front wheel axle to the "crown". The  "crown" is the T shape of the fork (the lower one for a double crown model or DH type)
To be precise, you need to measure from the top of the crown where the bearing seats. This length is usually measured by millimeters for its precision.

Second step, you need to make sure of the steerer standart, there is mainly 1", 11/8" and a newer but less common type of 11/4".
1" is somewhat for "older" frame generation
11/8" or also called "Oversize" is the most common at the moment
It is possible to mount a 1" fork on a 11/8" frame with adaptor clamp to fit the stem, but not the way around (Oversize to 1" frame not possible)

Third step is to make sure that fork will fit the frame mecanically.
The steerer lentgh needs to be longer than your frame head tube. It is very important point to observe. You wont be able to mount the fork if the steerer is too short.
Measure your actual mounted fork steerer up to the top of the stem. If the new fork steerer you plan to buy is longer, it is no problem, it can be cut to the exact size needed. It is a normal process to cut the new fork steerer to needed length.

Then of course you need to check weither the fork braking system is matching your braking equipment.
Note that V-brakes and Cantilever type are sometimes different positionning. However, most MTB now are ready for V-brakes and international disc brake mount.
If you mount disc brake, check weither it is International type or Postmount type.

You may also need to look for  front wheel  hub spacing and axle diameter accepted. Some suspension forks  are designed to work with specific axle diameter and specific hub.

Lastly, you may mount non-suspension forks (rigid) into suspension designed frame as long as the fork match your frame required fork length.