Need help with a workout for someone who doesn't gain weight! what kind of diet should i have to complement this?

No magic bullet: If your energy output is less than your energy intake, you would gain weight. Fat and carb tend to make one gain weight than protein. Thus you can gain weight either by reducing the intensity or even more importantly the duration of the workout, or by eating more food. Question is why do you want to gain weight if you are of average range? Unless , of course, you want to become a sumo wrestler.