If the length of the array is not zero, you possibly want to have the user key in the first element of the array first (before the while loop), so that it can be compared to the second element to see if you should pick the first element or the second element as a potential candidate for the biggest value and so on.
Also, the line
if (b>c) c==b;
does not work. If b>c, shouldn't b be chosen as a potential candidate for the biggest value? Not least is that be careful with the distinction between the assignment operator (=) and the relational operator (==).