Rest. I've gotten tendonitis in my forearms before when I was rock climbing regularly, and the best solution is just to take a break from whatever you are doing that aggravates it until it heals. I pushed it for a while, and now I believe that I have noticeably less grip strength in the affected arm. It might also be a muscular imbalance, and not an aggravation of the tendon- in which case in addition to resting your forearms, you might also try to do some strengthening/mobility exercises... i.e. if it is a problem with your flexors, try strengthening your extensors, and vice versa.
"It’s not about how strong you are, it’s how well you can move with that strength."