What's the whole story? Are you living at home, under their roof, on their grocery tab? Or out on your own? Either way, be a mature adult (or, at least, pretend to be one) when explaining it to them. Don't resort to the "I'm 19, I can do whatever I want!" crap. Tell them you appreciate their concern, and you love them enough to do everything within your power to ensure that they don't lose you. Tell them about the safety course, tell them you are always going to wear the proper safety gear (helmet, boots, gloves, armored jacket/pants - you are going to, right?), just be respectful.
And be safe, the R6 is not a toy.