The open enrollment period to sign up for health insurance using the marketplace has been significantly reduced for 2018 and will only last from November 1, 2017 to December 15, 2017. If you don't sign up by December 15, 2017 and don't qualify for an exemption then you will have to wait until next years open enrollment to sign up for health insurance coverage.
Common exemptions include losing your job or COBRA coverage, Medicare, Medicaid or CHIP eligibility, coverage for a plan that you purchased, or coverage through a family member.
The best place to sign up for coverage is through the official Healthcare.gov website. You will be directed to either take the first steps to apply if you don't already have coverage or to sign in to your current account. If you're signing up for the first time then you will then be directed to pick the state you reside in to begin the process of signing up for coverage. If you already have coverage then you can sign in and research if there are any potentially better plans for you or your family.
The sign up process is as easy as it's ever been, so don't procrastinate until it's too late to sign up for coverage.