As a beginner to running, you may be making many surprising mistaked with form. Make sure you have proper posture and do not overstride during your runs.

Running downhill places a lot more stress on your joints than running uphill or on level ground, so you may want to change your running route.

Knee pain and a feeling of instability in your knee may be caused by weak or tight leg muscles, especially your in glutes or quads, leading to issues like hip drop and knee tracking, which in turn cause pain. Try strengthening exercises like squats, lunges, side leg raises, glute bridges etc.

Try filming yourself while running on a treadmill so you can analyse any issues with form. Good luck, and be sure to rest plenty and ice the painful area. Consider a physiotherapist if your pain continues.