How do I seal a pool leak - Pool & Spa

Pretty vague question.

Vinyl, plaster or fiberglass pool?

If Vinyl and you located the origin of the leak, use a vinyl patch kit.

If Plaster, there are underwater curing, 2 part epoxy putties available, but this like the above patch, will be a "Band-Aid" repair.
Better way: drain pool to the crack, hole or whatever sacrifice in the shells integrity, and then cut/enlarge the leaking area and use a white "Hydraulic, non/minimal shrink Mortar".

If fiberglass, use a fiberglass repair kit.

Now if you do not know where the leak is, that is a whole different arena. Do you know how much water loss you have?
Does it leak more when pumps are running?
Is water loss the same amount if pumps are off?
Any moisture around the equipment?
Any mushy areas in the landscaping between the pool and equipment?
Good luck