Step One: Calculate Your After_Tax Income. Your after_tax income is what remains of your paycheck after taxes are taken out, such as state tax, local tax, income tax, Medicare, and Social Security. If you're an employee with a steady paycheck, your after_tax income should be easy to figure out. Look at your paystubs. If health care, retirement contributions, or any other deductions are taken out of your paycheck, add them back in.
Of course, now you must differentiate between which expenses are "needs" and which are "wants." Basically, any payment that you can forgo with only minor inconveniences such as your cable bill or back_to_school clothing is a want. Any payment that would severely impact your quality of life, such as electricity and prescription medicines, is a need.