I want it so when you go up to a mesh of ladder and press a button your character will get on the ladder. Then if you press w they will go up and same with s to go down. How would I go about doing something like this?
Depend how you have done your control.
If you have it only in “player controller” then just do other for when they are in ladder.
If you have the controls in your player blueprint, then do a check.
So when the player overlap a ladder and press a button, change how your controls work. In this case W just will increase your Z and S will decrease your Z.
If you still don’t have practice with overlap events, practice it first, later practice how to change controls and finally you will have this 
I would do a ladder trigger “zone” in front of the ladder.
If you enter that zone, you enable ladder mode.
First at all is know how to place triggers. Here is a tutorial:
After it you must do two events: begin overlap and end overlap.
In “begin Overlap” you must activate the secondary controller setup (that will depend where you have them, mostly cases that I saw so far are in Player blueprint). You can do this with a Boolean.
And in “End Overlap” just must use again the boolean, but just restoring his value.
The most common trick for making an object climbable is by using a material shader. Since all materials in UE4 supports a physics material extension you could make a physics material “ladder” and on surface collision the player could be made to climb.
The advantage it does not matter how small or how long the ladder is and if applied to the rungs could be used as IK targets.