You have to push hard on the brake pedal and push on gaz pedal (not full you risk to move before the green) and release the brake when green arrives push also full on gaz pedal, after some trials you can have a very good strart
What @alp776 says is true, the best you can do is to brake while hitting the throttle, right before you think the race will start, slow down so you dont hit the limiter when accelerating. Braking while hitting the throttle is a good thing aswell to warm up brakes and tyres for T1. The worst thing you can do is to hit the throttle all the time, being on the limiter and just "wait" for green.
So you need to be off the limiter and then hit the accelerator just before the lights turn green? Does it matter if in 1st or 2nd gear?
Do you really need to hit the brake before you get on the throttle?