I started a new job last January with a W 2. I filled it out the same way I did my other W2 job. Yet this year I owe almost $300! Why is that?

Since the tax legislation of 2017 that stopped using exemptions on returns, the Form W-4 process has become more tricky. 
 

TurboTax has a tool called TaxCaster to give you an idea of your tax liability and/or refund. You can vary different items and see how that could impact your return. You can try it as married filing separately, if you got a bonus, or sold investments.  This will give you an idea if you should make estimated payments or change your withholding for your W-2.  Click for TaxCaster.  If things change during the year, you can try it again and always make an estimated payment then. 

This could be helpful if you wish to check your Form W-4 withholding. You can file a new one at any time with your employer and TurboTax has a tool to help calculate the best withholding. Click for TurboTax Withholding Calculator.

Click for the IRS Payment page where they can take payments for estimated payments (or amounts due on returns). Please be sure to specify which type of payment.