Jazz is about learning standards/songs(like most music), learning theory, and learning to improvise.
Learn songs. If you learn a bunch of jazz standards then you can go play with other jazz'ers. Because you know the songs you don't immediately have to know any theory or know how to improvise. Depending on the people this could be enough. Why?
Because jazz is about taking a set of changes(lead sheet) and melody and then doing something interesting with it. So if you know the melody and just play it, that is interesting enough, because it if wasn't, the song wouldn't be a standard/popular. Of course, some snotty jazz'ers(which most average jazz players are) will look down at you because you are not doing anything interesting, but who cares about them? You are there to learn and you have to start somewhere. If you find some nice people to play with, they won't mind, they know where you are coming from because they went through the same stuff. In fact, those types of people will be better because they will actually understand what they are doing and usually won't mind helping you understand too.
So, basically all the theory and improv will come naturally AFTER you learn songs on your own. You will probably have to learn theory so you can communicate but you'll pick up a lot with on the job training. But the job requires "5 years experience"(that is, knowing a bunch of standards). Luckily, with jazz songs, it is not obscene to bring charts/lead sheets... so, you just have to get proficient at reading them.
At first, you should just listen a lot. Learn the vibe that is going on so you can fit in with it when you are ready. The music is easy... the human aspect is what you have to get down. If you have experience playing in a rock band, then don't use that! You are not playing rock but jazz.. A different vibe because it is different type of people(people who are more in to technical and intellectual aspects of music rather than sex drugs and rock and roll(although the first 2 still apply more or less)).
Start working on standards. Start learning how to read lead sheets. There is plenty of info on the net about this.
Note that you will be required to know a bunch more than basic music. Again, all the theory is on the net. Learn your guide tones because they let you play simple and stay out of the way. A big part of playing with other people is learning how to fit in... usually it involves less than more.
Learn to play all the extended chords(altered chords that don't show up in most other styles of music).
Learn the melodies(memorize them by playing them over and over, or at least get them under your fingers. If you play "wrong notes", it's jazz! you can get away with it. Just nod your head and smile! No one will know if you did it on purpose or not). The melodies are the basis of the tune, while you could play something completely different, chances are you'll suck at making up stuff because you can't improvise yet, also people are expecting to hear the melody as that is what makes the song... so not only do you have to play something that sounds good, it better sound better than the original melody... which again, chances are you can't do.
Start learning theory. E.g., what are substitutions and where can they be used? The ii-V-I(simple and almost all jazz progressions are based on this progression in some form or another. At least 90% of any succession of chords in jazz will be part of some ii-V-I even if it doesn't look like it).
By making the ii-V-I instinctive(through repetition = learning songs and playing around with them), the music will make more sense. Remember, you are trying to learn a language. Theory is the grammar. It helps make sense when your instincts fail you.
Start using your ear and connect it with the theory. This helps you start learning to think on a higher level. You stop thinking in terms of abstract sounds and more in terms of symbols and formulas... these are much more concrete than just hearing something and not knowing what the heck it was(when it was just a A7b9b13 chord).
Start learning to improvise. This is the easiest of all! Just start making up shit. Doesn't matter. Who cares! It's jazz! If anyone is condescending to you, smack them in the face and tell them to shut up and go listen to Bob Seger or the Ramones.
Really, the improv will come naturally because of the way things work. Don't sweat it. Most of the time people that can't improv is simply because they haven't done the previous steps well. To be able to improv you have to know how music works and you learn that by learning music. The theory helps you learn music faster than you could without it. (People that never learned theory but are masters could still benefit from knowing theory. It's just icing on the cake and never takes away unless you let it.)
So, start with step one. Give it some time. Learn at least 20 standards inside and out on your own. By that time you can start to learn theory by looking on the net(tons of info). Just start reading... never stop. Then start improvising a little(get some backing tracks to play over or play over the tunes and just make up stuff along with the soloist... maybe try and make interesting phrases along side his)... Give that a few months. By that time you should be in good shape to play with others. Say, give yourself 6months-1year to accomplish this stuff(you have to work on it every day). By then, you shouldn't have any issues. Just go find some people to play with(ads, community college, etc) and let them know you are new. Then just sit back. Your practicing will take over and you'll start to have fun! Might take another year to get where you want to be right now but it will come with patience and practice.
Ultimately the journey starts with the first step. When you take that step, and decide you are going to go to your destination and nothing will stop you, you will get there. But every step is the first step because you have always been on your journey. Master the first step and you will be at your destination! (remember, step one is learning standards!)