Allotment of centres and slots will be done randomly on basis of given preference list of centres shared in Jeemain applications form. 

Authorities shall declare slots as on 15th November 2018 apparently. 

All dates and details shared below. 

https://engineering.careers360.com/articles/jee-main-exam-centres

Hello,
National Testing Authority (NTA), the exam conducting body of JEE Main will be allotting the date, shift and timings for JEE Main 2019 January exam randomly to all the registered students. 
Candidates will not be able to do the slot booking for the exam. 
The admit card for JEE Main 2019 January session will be made available online from December 17, 2018, onwards. You can check all the information regarding the examination venue, slot, date, timings, etc. from the admit card.
Good Luck!