When doing an online job how do the taxes work?

Pretty much the same as if you go to the office. If you are an 'employee' you will file a W-4 with the employer and he should withhold federal and possibly state taxes and send you a W-2 at the end of the year. If you are an independent contractor, the employer will send you a Form 1099-Misc at the end of the year (and a copy to the government) and you will pay the taxes then. Most independent contractors should be filing estimated taxes quarterly to avoid a penalty when filing.