It depends on the calculator you are using. Some calculators simply use the division sign button (either ÷ or / ). Some calculators have a button that says a b/c.
If you have an a b/c button you usually have to press it between each number in the fraction. For example to enter 51/2 press 5, a b/c, 1, a b/c, 2, enter. To enter 1/2 press 1, a b/c, 2, enter.
If you have the calculator instructions or user's manual you can probably look in it. Otherwise try a simple calculation you already know the answer to using one of the buttons mentioned above and make sure you are inputting the fraction correctly into your calculator.