I guess you're not following any diet...
One of my recommendations is good exercise, to distribute what you might gain soon. Don't eat just to gain wight: eat balanced food that contributes to your physical development without damaging your body: balanced meat, for instance, can be the solution.
Some extra meals can also help you; however, these meals should be at specific hours, as well as healthy.
Your routine and state of mind can finally contribute to the success of your attempt to gain weight without drastic actions!