The first step we would recommend you take is to set up the bucket test. If your dealing with a slow to moderate leak setting up the bucket test is the best way to determine where the water loss is happening.
The Bucket Test determines if you are losing water from in the pool such as a hole in the liner or a faulty light gasket, from the equipment or underground plumbing, or if the water loss is from evaporation. Depending on your results we would recommend a dive or pressure test to further investigate the issue.
Click here to view video on how to set up a bucket test