If you haven't yet signed up for Obamacare coverage in 2017 or are looking to change your current plan and want to make sure your coverage beings on January 1 then you need to make sure to sign up for your plan by December 15, 2016. The final day to sign up for coverage is January 31, 2017.
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.
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.