Can somebody help with the below issue

I need to add 5 fields to a sales rep account. The fields are

1. Bonus (currency)
2. Elite Joining Date (date field)
3. Match Amount (Currency)
4. Match Earning (Match Amount * 2)
5. Total Stash (Bonus + My Stash + Match Earning)

The fields must be able to track history in case the amount is changed on a monthly basis.  For ex:
------------------------------
Month | Match Earning | Bonus
-----------------------------
Jan      | $50                  |  $160
Feb      | $70                 |  $150


I was able to add to add the fields. But was not able to find how to track the changes on the monthly basics?

Thanks