Is "Devilman: Crybaby" related to the 90's or the older Devilman series? I looked at MyAnimeList, there is a classic series, it's Devilman, had an anime adaptation in January 5th, 2018. I am interested to watching this series. Quick search for these series, it has a lot of adaptations since 1970's. 

Do I have to watch all of those serieses before started to watch Devilman: Crybaby? Are these series unrelated to the past series?

Note: I have never watched or tried to watch this series, so I have no idea about this franchise.
Yes

Devilman Crybaby roughly follows the plot of the original devilman manga (1972) which started the Devilman Franchise where there are sequels, prequels, remakes, new plot lines, crossovers ect... Essentially a continual flow of new content from the 70's to date, see the above link for details on which series fit where.

EDIT: Just commenting on the other answer saying No. My answer was really intended to respond to the title of those works being related where it seems you are responding specifically to if OP should watch something else first. While I agree Devilman Crybaby stands on its own as a story and there is no content that is necessary to see before or after watching... There are plenty of story connections to be found other works such as the manga titled "Devilman Shin" and "Devilman Lady" which are official sequels, or the prequel "Demon Knight"