Skydiving mechanics

Hi, I am pretty new to Unity and I am trying to make a game dedicated to skydiving (inspiration for Just Cause ). I have got a 3D terrain and everything up and running, with my character being able to move and jump. However I would like to make it so that whenever I jump, and press the jump key again, I go into some kind of skydiving mode, that slightly nullifies gravity and allows me to glide, and of like the batman glide option in the games. However, I don't know where to start! Can anybody give me some code, hints or clues on where to start? Thanks in advance