Best workout plan for ectomorph please help?

ok so heres the deal i'm 18 about 5'11 and weigh about 123-127, i have 20 pound dumbbells and a perfect multi gym which includes pushups and chinups and everything like that. So my question is should i eat a lot and workout at the same time or gain weight first then start working out to turn it into muscle i'm very serious about this i want to gain atleast 10-20 pounds of muscle mass please give me your best ideas and best ways to make this happen, thank you.