If you're just getting started learning Jazz, there are a number of ways that you can prepare for your first lesson.
The first is simple, but many students fail to do it: Do your listening homework. You might want to read a book on Jazz history or watch a documentary like Ken Burns's Jazz. (View it as a starting point only: It is by no means comprehensive or exhaustive.) There are also a number of Jazz history text books and listening compendiums that you can get at the library. Again, none are perfect, and where one falls short, another may excel in some areas, and fall short in others. Jazz is a world laden with criticism and opinion. Two excellent books that drive home this point include Paul F. Berliner's Thinking in Jazz and Gennari's Blowin' Hot and Cool: Jazz and Its Critics.
After familiarizing yourself with what Jazz is and following up on your basic listening homework, you'll want to start listening to the basic repertoire that Jazz musicians play. Purchasing a copy of the REAL BOOK (link way down below) can help you get on your way. You also want to have basic reading skills that allow you to read simple treble clef melodies and play them. If you don't have this ability, you can study it on your own or with any "mom and pop music store" variety teacher. Be sure to specify that your goal is not to learn how to read scores -- only how to read and play simple single note fake book type melodies. To this extent, you may also want to get your hands on a copy of Hanon: The Virtuoso Pianist. This should help you out with your fingering and chops.
After this, you'll want to go over your basic triads -- major and minor. Then, practice adding sixths (the note a whole step above the fifth) to the triads. (This would be 1 2 3 4 fingering in your right hand on the piano. Keep your pinky free for the seventh later on!) You should be able to play these chords comfortably and instantaneously in all keys. When arpeggiated, you need to be able to play them up and down several octaves.
After this, you might want to try working on my book Chord Construction and Harmonic Mastery for Acing The Real Book. This is the point at which you might want to give me a buzz to help you out. Once you finish it, learning tunes in the Real Book should be simple. The follow up volume Mastering The Changes will help you get good at learning tunes.
As for starter tunes, two easy ones to start with are How High The Moon and Satin Doll. Both are in the Real Book (v.1)
If you can play your ii V progressions in all keys (covered in Chord Construction and Harmonic Mastery), most Jazz standards should come pretty easy. (Even Giant Steps consists of little other than major, minor and dominant chords.) The Blues are also very important. It is a pallet of many different improvisational styles. Students who study with teachers such as myself will explore them thoroughly in all stages of their learning.
Overall, if you are planning to study with me or almost any other teacher, here in a nutshell is the skinny of what you need to do:
With this, and the right teacher or coach, you should be off to the right start!
(note: If you get a copy of my book, you probably qualify for a free trial lesson, so be sure to drop me a line after you order it.)