Yes there is.
keep practicing you will get the hang of it!
go on game modes and go to nba blacktop
you play the game right
To dunk you have to sprint using RT and hold X when towards up to the basket.
I don't think you can but you may be able to do it in a slam dunk comp.
You have to have a high dunk rating. You can always under signature dunk choose the 360 dunk when you edit your player.
you have to run and then press y at the same time
Improve your dunk and vertical (myplayer mode), read the manual for controls.