http://www.nikonchat.com/18-nikkor-lens/
I broke a piece off of my Nikon Nikkor 18-55mm lens, replacement?
All this camera stuff confuses me, I am a beginner and I dropped my lens like a dumbass and a piece broke off of the part where it hooks onto the camera body.
So yeah, now my lens won’t stay in place.
Help? 
You cannot fix a camera lens by yourself. You’ll need to take it to a place that specializes in camera repair. A good camera shop in your town could give you a recommendation.
What you dropped is a very sensitive piece of equipment, not to mention very expensive. So it may not be as simple as replacing a broken part.
ADDED LATER: I can’t believe so many people are suggesting that you buy a new one. That’s like suggesting you buy a new car if you dent your fender. A repair will cost much less than a new lens!
Nikon News