How do I file my quarterly taxes for 2018?

You do not file a quarterly tax return, but if you need to make a quarterly estimated tax payment, you can make it directly on the the IRS site.  Keep good records of the payment you make so that you can enter it on your tax return when you prepare it in early 2019.  Your proof of payment will be your own bank/credit card records.

https://www.irs.gov/payments/direct-pay

Here are the blank Estimates and instructions…..
http://www.irs.gov/pub/irs-pdf/f1040es.pdf

The 1040ES quarterly estimates are due April 17, 2018, June 15, Sept 17 and Jan 15, 2019.  Your state will also have their own estimate forms.

Or you can pay directly on the IRS website https://www.irs.gov/payments
Be sure to pick the right kind of payment and year.....2018 Estimate

Or do you need help preparing them and figuring out the amount?