I’m ready to file my taxes for 2018, but I just recently learned that I could contribute to a Roth IRA for 2018. I’m capable of maxing it out but it’ll take time. Potentially filling the taxes first might even help me max it sooner, in such a way that I could start contributing earlier for 2019.
Roth IRA has no effect on your taxes unless you're claiming the Saver's credit.
Yes, as long as you do so before April 15.
You are permitted to contribute to an IRA of either ilk — Traditional or Roth — and have it count toward your annual $5,500 2018 limit until April 15th, 2019. This limit increases to $6,000 in the 2019 tax year.
This isn't really an urgent thing; Since contributions to a Roth do not in fact impact your tax liability for 2018 people rarely take advantage of the additional 3.5 months to make contributions to a Roth.