I think at the beginning you should just try to eat enough to not be hungry. It's likely that you will lose weight pretty easily at the start just from the diet change. Of course, if you have a binge eating issue, this might not be the best advice. But if you don't, just eating enough to not be too hungry between meals will probably work okay. Otherwise, there are some online calorie calculators. You can search for "how many calories should I eat". Try a few out and see what the consensus is.
Female, 5'3", 48, Starting weight: 163lbs. Current weight: 135.
Starting bench press: 30lbs. Current bench press: 75lbs.