Just like every other problem, in math or any other subject, you take the information you have, and the relationships you know between the information you have and whatever you need to find, and use them together to find what you need. I noticed that you didn't mention what information you have.
In general, the circumference of a circle is (pi) times (the diameter), or (2 x pi) times (the radius). For 'pi' you can use (22 / 7), or 3.142 , whichever is easier for you. Either one will get you close to the real exact answer.
If you don't know the diameter or the radius of the circle, but you know something else, like for example the area of the circle, then you've got a slightly more complicated problem. I can't cover all possible problems you may run into, because, as I pointed out earlier, you haven't described what information you already have.
