If you are serious about your weight gain you will have to change your lifestyle somewhat to do what must be done.
Although there are more items that could be included here, these 3 are probably the tops for skinny guys to gain weight.
Rest Getting enough rest is one of the big ones for skinny guys trying to gain weight.
Whether or not you have a fast metabolism, slowing yourself down will really help.
Stop any aerobic activity or sports you do.
This is burning up the calories that you need to help you get bigger, take a few weeks or couple months and just be lazy.
Get plenty of sleep, 8 or more hours a night would be ideal.
During sleep is when your body can really go to work on growing.
If you have a chance during the day, try to take a nap.
This will also help your body slow down.
Eating You will need to eat much more food than you are probably eating now.
Stop eating 3 meals a day and make it 6.
Space the meals about 2 ½ to 3 hours apart.
As you increase your calories check your progress, if you do not gain anything add more food.
If you have trouble getting down a meal try drinking a meal replacement drink.
Lifting If you are not working out with weights you need to start.
If you are new to it start off easy so you can prevent an injury.
You want to eventually get to where you are lifting a heavy enough weight that will only allow you to do 5 to 8 reps.
Use only a barbell and do the squat at least once a week.
Do these things and you will see progress.
Don't give up hope of ever getting bigger because it absolutely can be done.