Need help.... How do i convert this information into an index and the forcast the next year

Month Year 1 Year 2 Year 3 Year 4 Forecast
1 18,000 45,100 59,800 35,500
2 19,800 46,530 30,740 51,250
3 15,700 22,100 47,800 34,400
4 53,600 41,350 73,890 68,000
5 83,200 46,000 60,200 68,100
6 72,900 41,800 55,200 61,100
7 55,200 39,800 32,180 62,300
8 57,350 64,100 38,600 66,500
9 15,400 47,600 25,020 31,400
10 27,700 43,050 51,300 36,500
11 21,400 39,300 31,790 16,800
12 17,100 10,300 31,100 18,900
Avg.

Question

Need help.... How do i convert this information into an index and the forcast the next year

Month	Year 1	Year 2	Year 3	Year 4	Forecast
1	18,000	45,100	59,800	35,500	
2	19,800	46,530	30,740	51,250	
3	15,700	22,100	47,800	34,400	
4	53,600	41,350	73,890	68,000	
5	83,200	46,000	60,200	68,100	
6	72,900	41,800	55,200	61,100	
7	55,200	39,800	32,180	62,300	
8	57,350	64,100	38,600	66,500	
9	15,400	47,600	25,020	31,400	
10	27,700	43,050	51,300	36,500	
11	21,400	39,300	31,790	16,800	
12	17,100	10,300	31,100	18,900	
Avg.

anonymous · Answer

May be you have to take average for each month in 3 years and use that as the forecast for the 4th year

anonymous · Answer

sorry,take average for 4 years

anonymous · Answer

Vsahal3
how do i do that. this is what i did. yr1 the 3 highest numbers are
83200+72900+57350=213450
then i divide it by 3
213450/3=71150 and this was the average for yr1. I did this for each yr. but from there i am confused. 
would the index be the average for all four yrs?
like this: yr1 71150, yr2 52743, yr3 64630, & yr5 67533

anonymous · Answer

For example for Month1of year5 it would be  (18000+45100+59800+35500)/4=39600
This is done for all the months