The Joint Admissions and Matriculation Board (JAMB) has not commenced the sales of the 2019 Registration forms just yet. Seeing as the 2018 forms were made available for sale in November 2017, my best guess is that the 2019 forms will also be made available on sale come November 2018.
Information as to when the 2019 forms will be available and when they are actually available will be updated here. Visit this site often to be among the first set of people to be privy to the 2019 JAMB Form sales information.