In scientific contexts, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal.
SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilo-calorie" and has the symbol kcal.
Colloquially, and in nutrition and food labelling, the term "calorie" almost always refers to the kilogram calorie.
This applies only to English text; if an energy measurement is given using a unit symbol then the scientific practice prevails there.
A convention of capitalizing "Calorie" to refer to the kilogram calorie, with uncapitalised "calorie" referring to the gram calorie, is sometimes proposed, but neither recognized in any official standards, nor commonly followed.
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).
Once you know the number of calories needed to maintain your weight, you can easily calculate the number of calories you need to eat in order to gain or lose weight: