How to gain weight the healthy way?

I am 21yrs old 6ft. 130 lbs. The fluxs from 128lbs to 135. I am a runner, varsity soccer player tall lanky guy. I need to get my wieght up to at least 140lbs for requirements to get into the air force. Whats the healthiest way to gain weight? I eat rly healthy and i dont weight lift just cardio. Ive been told many things from gaining wieght before lifting. To just loading calories. Also been recomended to drink ensure, as well as gnc mass xxx weight gainer. Open to sugesstions. Just eating tons of fatty food and doing nothing isnt for me though lol. Ty in advance.