It sounds like you're a true Ectomorph, so not much you can do to change your natural Somatotype.
There are 3 main Somatotypes (body types). Mesomorph, Endomorph and Ectomorph. Most people are a combination of at least 2 types. A true Ectomorph would tend to be tall, thin with long limbs and will find gaining weight difficult or impossible.
Maybe look at what you are eating, perhaps you're not eating enough? The average man requires 2500 calories per day to sustain current weight. Also try going to the gym, start doing some weights to increase your muscle mass. It's unlikely you will ever get much bigger though, but your overall shape may improve.
Bear in mind, if you start doing weights, you will also need to increase your calorie intake, more protein for muscle and Complex Carbohydrates for energy, so you may need up to 4000 cals per day.