What do you mean by "make it work"?
Using your function in the following scenario seems to work.
#define PI 3.1415926535897932384626433832795
double mycos(double x, double eps)
double s = 0.0
double an = 1.0;
int n = 0;
while (fabs(an) > eps)
s += an;
an *= - x * x / (2.0 * n - 1.0) / (2.0 * n);
printf("cos(60) = %lf\n", mycos(PI / 3, 0.1));
And the result is
cos(60) = 0.451689
It seems to be more or less correct (with a certain error level).
You can test it with various inputs and see where it makes mistakes (for instance instead of using PI / 3 radians, using 60 radians returns a wrong value). (Note: first I thought it calculates in degrees, hence I used 60 radians -- which I wrongly thought was 60 degrees and that's how I found this input to produce erroneous output.)