Im 19 also and 5'9 usually 150-160lbs and im repping 230 right now. I would suggest starting at around 130 or 140 , and attempting as many reps as you can while doing around 5 sets. Thats where I started and have added weight every week. Also, as soon as you finish a set do 10 elevated pushups. It hurts like hell sometimes, but it definetly works.