what you need to do is get the input from the user and put it in to the array of integers and then use a loop to go through and find the highest, lowest, and total. So set higher to 0 then in your loop use
if (marks[m] > higher) higher = marks[m]
same for lower where lower is intially set to the max size of an integer to start.
if (marks[m] < lower) lower = marks[m]