How could I create a stamina bar. I keep on trying and nothing works.

I want to have a stamina bar with which it's max amount is 100 and when you hold down left shift it goes down by 3 a second. When it reaches zero you can not go down anymore and it starts going back up at the rate of 3 per second. I can't seem to do it no matter what I do. Can somone help?I already know how to make it so you can't run when it gets to the point of 0 ,but nothing else I can do correctly, maybe I could get the correlating bar that changes width with the amount of stamina that you have.