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.
If you're self_employed, your after_tax income equals your gross income less your business expenses, such as the cost of your laptop or airfare to conferences, as well as the amount you set aside for taxes. You're responsible for remitting your own quarterly estimated tax payments to the government because you don't have an employer to take care of it for you.