The tax system is pay-as-you-go. With an employee, wther are amounts withheld from each paycheck. A self-employed individual has to pay estimated taxes. Your SE taxes are 15.3% of the profit. Your Federal tax is according to your bracket.
If you wait until the end of the year, you will probably face an underpayment penalty. You are expected to make four quarterly payment and keep track of those payments yourself. You will pay with Form 1040ES and enter the amounts paid on your tax return the following year.
