I can only tell you how I do the starts even if I don't no if I'm doing it perfectly right, but maybe it'll help you.
- I use manual shifting with Gamepad
- I always use full throttle
- when the lights switch to green -> 1st Gear, if to stay at full throttle or not depends on car and Setup.
While driving a low hp car as XFG for example, full throttle due to the whole start works for me but as mentioned before, dunno if there is a better way to start
- Remember: Full throttle after green lights only with low hp cars. When driving FZR for example, you have to decrease the throttle a bit till you have the feeling that your wheels are getting some grip. Practice that!!
Hope this will help you