I am going skatin later and I have osgood schlatter disease. Do I need a brace in case I fall?

A pad could help. Osgood-schlatter is a stress injury to the tibial tuberosity growth plate where the patellar tendon attaches. A fall directly on the tuberosity may aggravate your condition, so yes, a knee brace/pad of some sort may protect the area. Generally speaking, I use a strap across the patellar tendon instead of a brace to relieve symptoms associated with os.
No. Braces for osgood schlatters are primarily for symptomatic control and they are generally a strap that goes under your kneecap. I saw that you are 18 so technically your osgood schlatters is now adult jumper knee (same thing just not involving a growth plate anymore).