Begin by listing your income for the month. This should include any paychecks you receive, as well as income from other sources like child support or investments. If you have a business, you should include the amount that you pay yourself out of the business each month.
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?"