What should I do first if my pool skimmer is leaking?

Replace the rubber gasket between the faceplate and the skimmer body. That fixes 90% of skimmer leaks, takes ten minutes, and costs under $10. Turn off the pump, unscrew the faceplate, swap the old gasket for a new one, and tighten screws evenly by hand. If that doesn’t work, check the pipe connections or the skimmer body itself.