I'm not giving you the code: your lecture notes will cover that.
But, I will give you the steps you need to go though. It may be worth implementing and checking each stage first, before moving to the next.
1) Declare your sales data: You need somewhere to store the sales information, so your code can access it. I would use a Dictionary<int, int[]> so that I could use the year as a key.
Dictionary<int, int[]> salesByYear = new Dictionary<int, int[]> {
{1990, new int[] { 10000, 20000, 30000, 40000, 50000, 10000, 10000, 10000, 10000, 10000, 10000, 10000}},
{1991, new int[] { 9000, 3000, 4000, 5000, 6000, 3000, 2000, 5000, 4000, 7000, 4000, 4000}}
};
Gets you started!
2) Generate your totals: Again, you need to store the totals. Again, I would use a Dictionary. Fill the totals in your code - loop though each years sales, and add them together. Put the sum in a dictionary entry for that year.
3) Find the best sales year: Loop through each year in your totals, and find which is the highest.
4) You then need to sort the month numbers into order, with the sales value per month.
Good luck! If you do get stuck on some particular part, ask again, but please, do try to do it yourself!