Check the caulking outside, especially near the bottom. If any pin-holes or gaps or hard caulking, replace it all. If the sliding unit, check the rubber weather-stripping where it touches the fixed unit when closed. These sometimes wear, get brittle, or come out of their spline. If the water leak is from underneath the threshold, you will have to remove the whole unit, in order to install sticky window flashing and caulking (3 beads) underneath. Spend some time to be certain where the water is coming from. Then get back to us...
Be safe, Gary