Adding a weight gain product such as below would certainly be a great foundation. Adding nutrient rich additional calories to your daily diet can only be beneficial to your end goal of lean, quality weight gain:
Popular choices include Optimum Nutrition Pro Complex Gainer and BSN True Mass. A full list can be seen here.
On top of this, to help you find answers to your questions, you will definitely find it useful to take a read of some of our most popular articles on the subject:
Hope that helps. Enjoy!