I always wanted to learn more about jazz guitar and I've never found a good point to start learning it. I've been playing guitar since I was 12 (now my age is symmetrical to 12) and I am a self taught guitarist. I don't know much about theory and scales but I think I'm not really bad at learning by hear. I would like to learn some jazz stuff so that I can improvise something or even write something. Can you suggest any web sites, songs or methods in order to make my wish come true?
Jazz guitar is a broad church, so I'm not sure you can learn it, or get it, as a single entity. You could figure out how to fake it in a couple of different styles, but a self-taught guitarist will potentially be approaching everything from a wrong angle. To understand jazz you need to have a solid understanding of theory. You should be able to play every type of scale in every key, and be able to flow through keys. Imagine you have sixteen bars to improvise in a fluid, musical manner that gets you from key A to key B, through key C, and you have to do all of that without treading on the toes of your fellow musicians! You need the theory there to be able to do that.
One of the greatest teachers of jazz guitar out there is Ted Greene. Watch his masterclass videos and learn his chord melody transcriptions.
Browse a library of his handwritten lessons (many published posthumously) at http://tedgreene.com/teaching/default.asp
Before him, everyone had to wear out the grooves of their Django Reinhardt, Joe Pass, Eddie Lang and Tal Farlow records and try to learn everything they hard by ear. That would be the other tried and true method.
You can check David Reed's book "Improvise for real". It is about improvisation and not just guitar- or jazz-guitar-specific, and it is using some simplified music theory. The author argues that the music theory taught in schools is overly complicated and offers an alternative method to learn it, that is based on listening, singing and improvisation (or this is my understanding). I am a beginner guitarist (with much less experience than yours) and I do not consider myself a "musician", but there are some very positive reviews on Amazon from professional musicians playing different instruments -- and this is why I am recommending it to you. There are some reviews that say the book is a scam, but you can read them and decide for yourself (there is also some free material on the website, which was helpful for me to decide whether to buy it).