The 2017 Tax Cuts and Jobs Act (aka Tax Reform) doesn't eliminate the Affordable Care Act penalty until tax year 2019. Until then, the ACA penalty still applies to 2018 returns. This will be the last year for the penalty.
The penalty for not having health insurance (aka Shared Responsibility Payment) only applies to the months you didn't have health insurance.
For 2018, the penalty is either 2.5% of your taxable household income (up to the national average annual premium for a Marketplace Bronze plan) or $695/$347.50 per uninsured adult/child under 18 (up to a maximum of $2,085), whichever is greater.
If you tell us you didn’t have health insurance coverage or only had it for part of 2018, we’ll create Form 8965, calculate the penalty and also check to see if you qualify for a health coverage exemption.