void heapsort( int num[], int N );
where num[] is the array to be sorted, and N is the number of integers in that array. This routine should put the sorted integers back in num[].
Once you have this done, try generating a bunch of random arrays of N integers, and timing your routine for various values of N. For example, you might generate 1000 random arrays of size 100, sort them, timing the process. Then generate another 1000 random arrays of size 1000, sort them, and time the sorting process. Then do 2000, 10000, etc.
To generate random arrays and time the results, you need some tools. See here and here for examples.