How to deal with trust in this situation.

Ok, so I told my girlfriend I quit smoking. I told her I did but I didn't really want to so I went asking for some smokes from all my friends. But then one of them found out I told her I quit and then told her I didnt. I know I made this situation worse...

by lying, yes I realize that. But how do I fix this and gain her trust back?

Start by solving the issue that caused the problem. quit smoking if thats what she wants.
Dont lie anymore and give her proof of the things you tell her, but do it stealthfully, as you dont want her to think that you see her as over bearing or controlling.
She will begin to trust you as you come through for her and quit lying.