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?"
List your expenses starting with the most important to the least important. Listing the expenses in order makes it easier to make cuts in your budget if needed. The most important should be things that cover your necessities. You will need to cover your housing, food, utilities (not including cable television), transportation costs, debt payments, and savings goals first. Then you will list luxury items which can include clothing, entertainment, eating out and gym memberships.