When dealing with a leaky pool first you have to acertain if it is vinyl or cement. If vinyl you can use a patch kit like for a bicycle. If it is cement, you will have to lower water level and use a epoxy, let it dry and refill.
in most cases you will have to purchase a pool patch kit. Its very simple to do. Cut out a small piece of the patch material and apply to the inside of the pool
First, drain the pool. Then try applying duct Tape as that is an easy fix and refill the pool to see if the leak is fixed. Otherwise, call the pool repairman.
The best way to avoid leaks is to spread a plastic ethylene sheet between the groung nd the concrete. But since you already built it try plugging with M-SEAL. It holds better in wet conditions than other materials.