Even though there is a lot of talk about repealing the Affordable Care Act (ACA, aka ObamaCare), the ACA is still the current law of the land. Which means changes taking place were already scheduled to do so.
One of the major changes you need to be aware of is the fact that the Enrollment Period is shorter for 2018. The enrollment period is scheduled to run from November 1st through December 15th. Outside of that enrollment period, one will have to qualify for a life change event to be able to opt in for coverage.
See if you can qualify here: https://www.healthcare.gov/screener/