Your post doesn't offer much information about your training or diet. And your profile says you're well over six feet. So, you have a lot of work ahead of you if you want to fill out your frame with lean muscle.
Gaining weight isn't hard, but gaining *muscle weight is. There are plenty of guys on here who are personal trainers and can give you solid advice. But here are a few tips of mine.
Eat lots of quality foods, with a balance of protein, fat and carbs. Six smaller meals will give your muscles the nutrition they need to grow.
Structure your workouts so you don't overdo it. Mix it up a little. If you do the same routine every time you go to the gym for years on end, you will make little progress. I'm sure you've seen guys at your gym who never seem to make any gains. Your body will always seek stasis. That means, when you break down muscle tissue by lifting, your body will flood itself with chemicals to undo the damage. Stay with the tearing down, and, little by little, your muscles will grow. But change your approach every 6-8 weeks.
The reason pro bodybuilders get so big is that they use steroids to chemically inhibit the body's natural tendency to keep muscles from getting too big. Regular guys, those not using steroids, have to rely on training smart, letting your body rest between workouts and getting plenty of sleep.
Try scheduling workouts so that you hit each bodypart just once a week. Too many younger guys think if one arm workout is good, three are better. No. And if you're doing a lot of running, stop. Also, new research suggests you do any cardio exercise after your workout and not before.
Get yourself a blender and make your own protein shakes. Overall, it's cheaper. Add a couple to your daily diet, but slowly at first. Intestinal upset can occur if you start eating a great deal of food all at once.
Drink plenty of water. And, no, a daily vitamin does not count as supplementation.
Finally, in bodybuilding, patience is a virtue. If you're serious about getting bigger muscles, be prepared to spend a couple of years working out. Your height is a disadvantage. It will take you longer to fill out your frame than a shorter guy. The good news is, any gains on you will likely be more apparent.