Ok, then the best and easiest way is to go to:
Deductions and Credits
I'll chose what I work on
scroll down to Retirements and Investments
then the first one is Traditional and Roth IRA contributions.
You can enter what you are thinking about contributing and see the tax impact (if a Traditional IRA)
Then you just need to make the contribution to the IRA by April 15, 2019 and the contribution must be designated as a 2018 contribution.