2018 1st quarter estimated taxes are due 4/17/18. 2017 tax return is also due 4/17/18, although you may file for an extension to file. The extension allows you 6 more months to file your 2017 return.
However, it does not extend the time to pay any taxes due (including an underpayment penalty if applicable). If you have a 2017 tax liability, it is due 4/17/2018. You can pay the 2017 tax liability and the 2018 1st Quarter estimated taxes directly on the IRS website.
See the link below for the 2018 estimated tax vouchers.