In order to gain muscle you have to eat more calories than you burn off. This is a common mistake that people make when trying to build muscle. When you first start muscle building you need to remind yourself each day to eat, so that you get into the habit of eating frequently. This will help you reach your target number of calories.
So say you are a man who is trying to gain muscle, you should be aiming for around 2700-3200 calories which will provide you with enough extra calories for muscle growth whilst minimising fat gains. You may have read in some places that you need to eat even more calories than this to build muscle, but this is far from the truth, you don't need so many calories that you gain a substantial amount of fat.
5-6 meals a day which are about 3 hours apart from each other can provide you with a regular intake of nutrients. It's important that you remember to keep eating and don't skip meals. Skipping meals would be one of the worst things you could do since your muscles are craving calories if you have worked out that day or the previous day.
Sometimes it may be hard to reach 5-6 meals in a day especially if you have a busy lifestyle. However, not all of these meals have to be a large plate of food. Protein shakes that have enough calories in them can be a useful way of getting a quick dose of protein and carbohydrates between your large meals. Nuts such as almonds and cashews can be another easy method of getting beneficial calories when you are at for example work.
Remember that your diet is as important as weight lifting if not more. Without feeding yourself with the necessary nutrients, you could work your socks off at the gym but never notice any real results.