Due in part to a culture who looks at abs as the end all be all sign of fitness, many are scared to eat. Get over it. Look, you are going to probably gain a little (notice I said a little) body fat. Caloric increases over time can do that. I'm not talking about the bloated "perma-bulker" strategy that is often employed by those not in the know. That free for all type of eating only leads to lowered insulin response, lowered testosterone and other not so desirable side effects. That guy gets fat. By employing smart strategies (calorie cycling, nutrient cycling, proper use of training variables etc...) you can get bigger with a minimal amount of fat gain.
It does require effort though, as all things worth having do. When in doubt eat. Here are some easy rules to follow to get to mass land:
1. Think protein first, fat second, carbs last.
2. You must drink copious amounts of water (a gallon or more depending on weight - remember muscle is 80% water!).
3. Go by the scale, but also the mirror, and definitely the caliper (body fat measurements).
4. Calorie cycle for lean gains and to keep body fat levels in check.
5. Use conditioning/cardio training to stay hungry/eat more - not to get in better "shape."
6. When in doubt, eat!
Use these tips as a guide on your journey. As I have often stated, "I'd rather miss a training session than miss a meal." Food is that important. Now go eat!