Try this Calorie Calculator....or this one. You enter some information about yourself (weight, height, age, activity level, etc.) and it will tell you how many calories you should be consuming on a daily basis (for weight loss, maintenance, or gain- don't know many people looking to do the latter).
One of the biggest mistakes I am finding is that people aren't eating often enough or consuming enough calories throughout the majority of the day, which leads to overeating or binges later. Typically, we should be striving to eat 300-400 calories every three hours to keep our blood sugar stabilized. Although you may feel like you're eating all the time (especially if you're only used to eating three big meals a day, every 5 hours or so), but you'll see that overall you end up eating less and feeling more full and energized throughout the day. Wild swings in our blood sugar actually slows down our metabolism (just as skipping breakfast does), suck our energy, and make us feel like poo. Rev up your metabolism today by eating a good, healthy breakfast (within 30 min of waking) and consuming a combo of lean protein and complex carbs (around 300-400 calories) every three hours.