The general answer to your question is you need to know the chord progression of any given song and be able to play those chords in time.

Usually at jam sessions people will call out the key signature, "Sweet Home Chicago in A!".  If it's a relaxed enough situation & there's enough time they might tell you the basic chord progression & form, "It's a I-IV-V 12 bar blues in A".  Some people carry a digital tablet or phone & can look up the lyrics & chord progression of a tune in the moment.  It used to be leadsheets or songbooks with the chords & lyrics.  :)  

You picked an interesting example ("Blue Moon of Kentucky") because there are quite a few different arrangements in various key signatures.  Everyone would have to know which one before you started.

For now just focus on building your skills.  Learn all the basic major & minor chords.  Practice strumming & switching between them in time.  In the Fundamentals courses you will learn a basic understanding of key centers, chord progressions.  Eventually you start to learn some tunes and commit the chord progressions to memory.

It's a long process.  Take your time & enjoy it.  Hope that helps!