What are your goals?

There are three main goals when it comes to fitness, gain muscle/weight, lose weight, and maintain. First, before you can start knowing how many calories to eat no matter the goal you need to find your approximate BMR. Your BMR is the baseline of how many calories your body burns each day without doing any extra activity. This will be an important number to know in order to find how many calories you should be eating a day to meet your goals. Gaining and losing weight is a very simple process that has been over complicated by people that don't know what they are saying. Weight fluctuation is about calories in and calories out. If you consistently eat more calories than you burn, you will gain weight, if you consistently eat less calories than you burn you will lose weight.

Weight gain

Weight gain is very simple, over time you will gain weight if you consume more calories than your body burns. How much you gain is based off of how much of a caloric surplus you are in. When trying to gain weight in a healthy way it is recommended you stay in a small caloric surplus, usually gaining around 0.5 - 1 lb a week, which translates to eating 250 - 500 calories more than your BMR each day.

Weight loss

Weight gain and weight loss are very similar, just in the opposite direction. If you are looking to lose 0.5 - 1 lb a week you need to be in a 250 - 500 calorie deficit.

Maintenance

Weight maintenance is probably the easiest of the three. In order to maintain your current weight all you have to do is eat the same amount of calories as your body burns. This means just calculate your BMR and eat that many calories a day and you will maintain.

Overall healthy eating

It doesn't matter if you are gaining, losing, or maintaining your weight, you should be eating healthy. Healthy eating means you are keeping a decent protein/fat/carb ratio and also eating fruits and veggies in order to get your body all of the nutrients it needs in order to function in the best way possible.