What should one do if they were just told that they have patellar tendonitis?

Acute or chronic?: It depends on the cause. If the tendinitis is acute and caused by injury the important treatment is rest. If you have chronic tendinitis it may require physical therapy, medication, and/or injection. If all else fails there is also surgery for some types of tendinitis. At your age there is a seperate problem which often looks like tendinitis called chondromalacia patellae. PT is its treatment

Stretch your quads: The most important part of patellar tendonitis treatment is getting on a regular treatment program that includes quadriceps (front of the thigh) stretching. Stretches should be held for 30 sec, gradually working up to 2-3 x per day. Straps can be helpful when placed right under the knee over the patellar and may allow earlier return to play