Since EOS is a distributed project, there is no one central official place to vote. There are several different ways to vote for Block Producers on the EOS mainnet.
The first is using the command line tool, cleos.:
cleos system voteproducer approve votername producername
The more user friendly ways involve a signing tool like scatter and a web site. One of the websites that allows you to easily vote is https://bloks.io/vote
