If you are already a supporting or attending member (or any other membership class with WSFS voting rights, which varies by convention) of the 2017, 2018, or 2019 Worldcon, you need take no further action at this time. You're already eligible to nominate for the 2018 Hugo Awards.
If you are not a member of one of those conventions, you need to join either the 2018 Worldcon or the 2019 Worldcon by December 31, 2017 to be eligible to nominate. Actual nominations will open in January.
These deadlines apply only to nominating. Voting on the final ballot is open only to members of the 2018 Worldcon, and the deadline for joining to vote on the final ballot is the end of final ballot voting. Those dates have not yet been announced.
This deadline used to be the end of January of the Worldcon year. A change first proposed and passed in 2016 in Kansas City and ratified in 2017 in Helsinki moved the deadline back one month to make it easier to administer the election. Changes in the past few years in privacy laws have made it increasingly difficult for Worldcons to cooperate in administering nominating voting. Each Worldcon is a separate legal entity, and in the case of the 2018 Hugo Awards, all three of the relevant Worldcons are in different countries from each other, further complicating the necessary cooperation. Worldcon is not a single centralized entity with a single membership list. Because you may only nominate once no matter how many of the three relevant Worldcons you join, the Worldcons must cooperate in a way that doesn't contravene their local jurisdiction's data-privacy laws to allow each member no nominate no more than once.
Note also that this is the final year that members of the following Worldcon are also eligible to nominate. A rules change first passed in 2016 and ratified in 2017 returns the eligible electorate to only the current and previous years' membership. The following Worldcon's members were added only a few years ago, and in practice very few of those members were not already eligible to nominate by dint of being members of one of the previous two Worldcons.
I'd like to point out that changes to the WSFS rules are not secret. Proposed changes take a minimum of two years to pass. They are published on the WSFS web site, and on the individual Worldcons' sites, and in their publications. If you are wondering what other changes may be in the pipeline for 2019, go look at the Business Passed On at the WSFS Rules page. Only items on the Business Passed On can first affect the 2019 Worldcon. New business at San José — and I expect a fair number of proposals to change the Hugo Award categories — if passed must also be ratified by the 2019 Business Meeting in Dublin.
I did forget to update the plain-text description of the membership deadlines on the Hugo Awards web site until today when it was pointed out to me that it still had January. That's regrettable, but not malicious (despite what some people seem to assume). It's updated now, and I also posted a "sticky" post to THA.org reminding people that they have to join by the end of this year to be eligible to nominate in 2018.
Rather than panicking over deadlines, the simplest way to make sure that you're always eligible to nominate for the current year's Hugo Awards is to vote in Worldcon site selection every year, even if you aren't planning to attend. Once you get started, you are (in effect) paying your WSFS membership dues two years in advance, and you're always eligible to nominate and vote on the current year's Hugo Awards without worrying about any other deadlines.