Categorize your purchases.
Look over all your purchase for the past year and define a category for each on such as Taxes, Car, Food. Then break those categories down. Taxes/Income Tax, Car/Gas, Food/Dining Out. Here is an example of an end result:
Car: Gas, Maintenance, Taxes
Now you can use those categories to find out how much you have spent on each specific category and how much you have spent on the overall category. Once you have compiled that list you can compare with other years and come up with an average amount you spend.
Purchase a computer program to help you.
Quicken, Microsoft Money, Mac Money and many others can allow you to keep close tabs on your finances. They will also break down your categories and allow you to create a budget relatively easily. The most valuable part of the program is the reports they offer you. You can build a report to show you your total amount spent on each category or whether or not you are over budget.
Answer budget questions.
According to Everybody's Money, a Credit Union National Association (CUNA) sponsored publication, if you can answer "yes" to most of the following questions, you have pretty good control of your financial affairs.