The Hugo Awards are selected by a vote of the members of the World Science Fiction Convention (Worldcon) in a two-stage process. The first stage is nominating (which starts in early 2019) and the second stage is the final ballot that includes those works/people that received enough nominations in the first stage (which starts later in 2019). If you want to participate in the nominating stage and are not yet a member of either the 2018 or 2019 Worldcons, take note of the December 31, 2018 deadline for joining Worldcon in order to be eligible to nominate in 2019.
If you want to nominate works/people for the 2019 Hugo Awards, you must be a member of either the 2018 Worldcon (San José) or the 2019 Worldcon (Dublin) by the end of 2018. (You can of course be a member of both, but you can only nominate once.) If you were a member of Worldcon 76 San José (supporting or attending, or any other membership class that included voting rights), you are already eligible to nominate. If you were not a member of Worldcon 76 San José and are not a member of Dublin 2019: An Irish Worldcon, you must join Dublin by the end of 2018 as at least a supporting member by the end of 2018 to be able to nominate.
To join the 2019 Worldcon, go to the Dublin 2019 Membership Page. You do not have to attend Dublin to be a member. A supporting membership includes all of the membership rights including nominating for the Hugo Awards, but not the right to attend the convention itself. The other membership classes including Attending and Young Adult can attend the convention and also have the nominating and voting rights. If you join Dublin as at least a supporting member by the end of 2018, you will be eligible to nominate for the 2019 Hugo Awards.
The final ballot will be announced sometime after nominations close. Only members of the 2019 Worldcon will be eligible to vote on the final ballot. The deadline for joining in order to vote on the final ballot is when final ballot voting closes. Dublin 2019 has not yet announced specific dates. The December 31, 2018 deadline applies only to joining in order to nominate, not to vote on the final ballot.
The 2019 Hugo Awards will be administered by the 2019 Worldcon. See the 2019 Hugo Awards web page on the 2019 Worldcon’s web site for additional information. Address specific questions about the 2019 Hugo Awards to the 2019 Worldcon. The Hugo Awards web site does not run the Hugo Awards. Each year’s Awards are run by that year’s Worldcon.
In general, the 2019 Hugo Awards (and the 1944 Retrospective Hugo Awards, also being run by the 2019 Worldcon) are for work first published in the previous year. Therefore the 2019 Hugo Awards are for works first published in 2018 and the 1944 Retro-Hugo Awards are for works first published in 1943. There are exceptions for works previously published in languages other than English, and for works originally published outside of the USA. See the section about “Year of Eligibility” on the Hugo Award Categories page for a longer discussion of this.