Im 19 years old. my height is 6'1" and i weigh, with just boxers on, 148 lbs. i don't look underweight but i can't seem to gain weight. should i worry?

Lucky you: That's a great weight for you. Ignore the subscientific "ideal weight" charts; every olympic track star would be underweight and every super-fit football linebacker "obese". You want lean muscle; you can build it by eating enough protein, avoiding grease, salt, sugar, and adopting a fitness lifestyle; resistance-train, creatine & an eledctronic muscle stimulator might be good too; ask your dr.