I am a teenager, thin and i eat a lot but don't gain much weight. is this normal?

Yes it can be normal: Healthy eating habits as a teen are extremely important for growth and development now and into adulthood. Being able to eat a lot and remain thin is a good thing, but you want to be sure what you are eating is healthy. Schedule an annual evaluation with your pediatrician to go over your growth chart, diet, exercise habits, etc to determine if further evaluation is needed.