Pick out an algorithm, for example Newton's method seems to be rather comfortable to compute the square root of a given number. This method is also identical to the ancient method of the Babylonian to draw the square root of a given number, it s also called the divide-and-average algorithm. http://www.homeschoolmath.net/teaching/square-root-algorithm.php discuss it. Try to design a c program. No matter whether it is complete, compiles with error, or produces imperfect results you should post your tentative steps. Then we will help you.
P.s. I don't agree with the concept of the latter post. The intention of computing the square root that way is not to do it plain "from scratch" but to learn fundamental concepts of numeric analysis, such as approximation, iteration, recurrence, numerical stability, and finally to get acquainted with famous Newton method.