Get your taxes done using TurboTax

No, commissions are not taxed at a different rate. If less tax is withheld, you will generally owe tax when you file your tax return.

On the prior years tax return (Form 1040), you can compare Line 18 with Line 25. Line 18 will show how much taxes you were required to pay and Line 25 will show how much you withheld during the year. This will give you an indication if you want to increase your withholding or not. However, keep in mind that increasing your withholding will lead to paying more taxes during the year which may result in a reduced pay check but then during tax filing time, if all the other things are same, you will owe less ( Exceptions apply ) . 

You can also visit Tax Withholding Estimator. This is a tool by IRS which provides guidance on how much you should withhold based on the information you enter.

Additionally, you mentioned that you claim 0 dependents on your W4. It would be a good idea to increase your tax withholding during the year so that you don't owe that much when filing taxes. This can done on your W4 by adding additional amounts in the "Extra Withholding" Section. I have attached an image with the highlighted area for further reference.