Proper diet for gaining muscle mass?

I'm an ectomorph so it is really hard for me to gain weight so I need a better diet for muscle gain but I want to be a "clean" bulking so I don't gain lots of fat. I need to get about 3000 calories a day but I'm lucky to get about 1500 calories a day with my current diet. I have been bulking now for about 4 weeks and i have been making some gains but nothing significant. Please tell me which foods are great for adding calories but not high in fat