The diameter is the length across the circle directly through the center. You can measure it in whatever unit is appropriate, such as inches or centimeters. For the sake of an example, say the diameter of your circle is 2.5 inches.
Multiply the number you have times pi. In the example, multiply 2.5 by pi. You can use the rounded off version of pi, 3.14, though most calculators have a button for the longer version of pi.
If you use your calculator's pi button, you end up with an irrational number because pi is irrational, meaning it cannot be stated as a fraction and the decimal continues on into infinity. Therefore, you need to round off your answer. Two places after the decimal is usually sufficient. You look at the third number. If it is above 5, you round up, meaning you add one to the previous number. If it is below 5, you round down, meaning you keep the previous number the same. In this case, pi times 2.5 is 7.853. Because the third number is 3, you round down, leaving it 7.85.
Add the unit you used to measure the diameter: 7.85 inches.