How do I pay for the 2018 taxes and how much do I need to add for the penalties and interest?
You, and only you can answer that question by actually completing your tax return. Nobody else can do it for you, unless you pay someone like a CPA to do your taxes for you. Even then, you would have to provide that CPA with all of your financial information and tax documents in order for them to complete your tax return.
I filed for an extension, which means I need to pay before October 15th.
No, that is not what it means. When you file an extension and the extension is approved by the IRS, that just means you have until Oct 15th to *FILE* your tax return. I(f you owe the IRS, then you have to "PAY" the IRS by April 15th. You flat out can not extend your time to pay unless you take physical action to set up a payment plan with the IRS for what you owe. You can't set up a payment plan until you file your taxes and actually know what you owe. During the period of the payment plan interest will accrue on your outstanding balance until it is paid in full. With rare exception, a payment plan can not exceed 12 months.
