Go on some videos on youtube, find something that you think you could do or want to learn, then look up tutorials on youtube.
After you got that part down, go outside, find an object that what you wish to practice could be practiced on, and go for it.
Me and my friends used to run around the train tracks jumping random buildings and climbing random things, then we got caught by the cops several times and decided to take it to the next level.
So we went around our neighborhood and tried out some parkour things that we saw in videos, eventually got better and better by practicing and moving on, and now are simply doing flips to add to the fun.
I don't really see parkour as a professional sport though or a profession, it just a hobby that keeps me busy during the summer. :)
Oh and practice somewhere with other beginners or away from the public. If someone is watching you it's a good idea to do something amazing. :P
(when jumping off of really high objects, landing crouched and absorbing the impact with your hands will hurt your hands, so a roll is recommended.)
Do not need muscles to do parkour. Just need speed and dexterity.
EDIT: Parkour is not as hard as people make it out to be.
Actually, for certain things you do need muslce. My friend is really weak, he can barely hold himself up in a handstand and can't do more than one pull up, if that's the case, I do suggest working on your muscles since he can't do certain moves very well such as cat leaping, double kongs, certain under bars, etc.