Don't be scared of carbs
Carbs have been demonized for a long time, but it's time to set the record straight. While it's true that Western societies depend too much on heavily refined carbs, that doesn't make all carbs bad.
The truth is that even vegetables contain carbohydrates, and getting plenty of healthy carbs throughout the day fuels your metabolism, gives you energy, and helps you recover from workouts. Aim to get a handful of carbs along with each serving of protein, which should keep you fueled. Just make sure you're getting your carbs from natural sources, not brightly colored packages.
Make sure you eat enough
Whether your goal is losing fat or building muscle, calories are king. If you're not eating to support your goal, you're spinning your wheels. If you're trying to drop fat, you need to be eating fewer calories overall. Which means if you're trying to build muscle, you need to be eating more; presumably, you're going to be working out more.
This is part of the reason why getting plenty of lean protein and carbohydrates throughout the day is so important. Not only do those foods help fuel recovery, but they're a surefire way to make sure you're getting enough healthy calories to support your muscle-building, fat-dropping goals -- and it's important to remember that not all calories are equal. If you don't want to jump up several shirt sizes, you might not want to eat an entire ribeye for every meal. If "lean" is what you're going for, you'll have to adjust your caloric intake accordingly.