If someone said they wanted to start playing jazz but weren’t sure where to begin, I would say:
Get the Real Book.
Look at the artist and album listed on songs in the Real Book and listen to those.
Go see live jazz whenever possible.
Learn 12-bar blues in all keys. The ones I’ve needed most often are (concert keys) Bb, A, G, E, and Eb.
Associate with a group that has at least one old pro, emphasis on “old” and “pro”.
Find out where there are jam sessions in your city. (Be ready for some late nights on weekdays.)