I have tendonitis in my elbow. What could I do to help the pain?

Stretching. Most tendinitis about the elbow subside on their own. Keeping the elbow fully extended and bending the wrist up and down will stretch the tendons and help with the pain. If this does not work sometimes an injection is needed. In severe cases when nothing else works there are surgical procedures to help.
Many options. Physical therapy for iontophoresis and phonophoresis, deep tissue modalities, stretching, and strengthening. Ice for 10-15 min 4 times a day, counterforce strap bracing, nsaids, and avoidance of the activity that aggravates. Occasionally a wrist brace is used and steroid injections can be considered for cases lasting months that don't respond. Surgery is rarely indicated.