My son always started in 2nd on 60, 80, 100, & 125. Don't shut off to find traction, use the clutch. Keep the throttle wide open and shift into 3rd without clutch as you approach over rev.
For mx 1st is a dead gear, you will rev out too fast, have little control if you are being as aggresive as you should be. If you mangae to keep the front down and the bike straight you'll probably light up the back and lose the traction.