What best way to treat a tennis knee injury?

Observe. Observe for something serious. Tennis knee implies overuse, and therefore not a serious injury. Rest, ice, anti-inflammories, light compression wrap and see how it goes. If you develop swelling, worsening pain, skin discoloration, popping, sticking, or other joint instability in the next week, then you need to go see your doc for xrays and possible referral to an orthopedic specialist.
Need more info. Hard to tell by your description. This could be many diff issues. Rec seeing an ortho doc for eval.