What to do if i felt a small pop in my knee playing soccer.?

See details: If you are experiencing knee pain or instability then see an orthopedist and have the knee evaluated. A tear is possible.

Depends: The answer depend on what happened after you felt the pop. If you then had pain and swelling, with or without clicking and limited range of motion, then you need to have your knee examined. If the knee was ok after the pop, then it was not important. Many joints/ligaments pop every so often with activity, similar to your knuckle joints, and popping once in a while is not important by itself.