Are you too thin out of choice or because that is the way God made you?
I battled anorexia for years. I was too skinny because I wasn’t giving my body the proper nourishment. If this is the case for you then I encourage you to confide in a loved one (possibly a parent) and seek the help of a doctor. The bible tells us our bodies are our temples. 1 Corinthians 6:19-20 says "Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your body." Because our bodies are our way of serving God and others we need to take good care of them.
If you are just naturally thin then I encourage you to find confidence in how God made you. He has a plan for you. He has created you exactly the way you were meant to be. If you are feeding your body the proper foods and you can not gain weight then that is not your fault. Trust that God will do what is best for you.