By listing all of your sources of income against all of your monthly expenditures (from required expenses like mortgage or rent payments to discretionary spending like eating out or going to the movies), you get a true picture of your personal cash flow, which will allow you to make better and more informed financial decisions. An accurate budget will also help you to answer that ever elusive question, "Can I afford it?"
Next list all your variable expenses_those that may change from month to month such as groceries, gas and entertainment. This is an area where you might find opportunities to cut back. Credit card and bank statements are a good place to start since they often itemize or categorize your monthly expenditures. Tip: Record your daily spending with anything that’s handy_a pen and paper, an app or your smartphone.