Yes just roll it over
When do I pay tax on it?
If you roll it over (convert it) you will pay tax in the year you make the conversion (2018) on the amount of the conversion (what is actually converted).
You still have time to recharacterize (make it as if you had contributed to a roth in the first place) until the tax filing deadline. Your IRA provider can help with this. If you've already filed your taxes you'll need to file an amended return. You would pay tax on the income on your 2017 taxes for the $5500 you're no longer deferring in a traditional IRA for 2017.
There is no marginal 12% bracket in for 2017. That bracket was introduced in the new tax bill which takes effect for the 2018 tax bill. If your income is staying about the same in 2018 as 2017 theres a pretty good chance your marginal tax bracket will go down in 2018 (from 15% to 12% for example).
Also consider the
savers tax when deciding between traditional and Roth.
Remember that what matters is your marginal bracket when you contribute vs your marginal bracket in retirement. You want to make sure you have at least some tax deferred (traditional) when you get to retirement so you can fill up the standard deduction and the lower brackets with withdrawals. A pension would also fill up those lower brackets.