So, you want to become a Jedi, wielding fantastic powers and a spiffy lightsaber, fighting the forces of the Dark Side and becoming one with the Force. It's a long, arduous journey, filled with terrible challenges, backbreaking physical training, and excruciating acting. Are you ready?
First, you need to ask yourself some questions.
Are you willing to subvert basic desires in pursuit of enlightenment? Can you be passive, rather than active? Can you be reactive, rather than proactive? Do you seek knowledge, rather than power? If your answer to any of these is "No," then skip down to the bottom.
Are you a young child on Earth?
Get a flashlight/cardboard tube/toy lightsaber and a cool cloak. Quote Star Wars incessantly. Have fun.
Are you on Earth, but not a child?
Just get a life.
Are you Force Sensitive? Clues may be that you have consistantly good luck, amazing reflexes, or get flashes of the future.
If no, you're stuck. See above. Otherwise, continue.
Look around. Are you living in the Republic? (A good way to tell is to ask around about a guy named "Anakin Skywalker". If nobody's heard of him, or people seem interested in talking about him, you're living in the Republic. If people turn pale, go to the next question.)
Go to the Jedi Council on Coruscant, although talking to any Jedi Master should get you a good start (as well as a sponsor, which should help.) In any event, you'll end up at a Jedi Academy, learning basic skills in the Force, in addition to core knowledge about the Republic and its history. Think elementary school with extra lessons in telekinesis.
After graduating from a Jedi Academy, you're eligible to become a Padawan, an apprentice. Put up with incessant cluelessness of your Jedi Knight or Jedi Master trainer, as most of them have spent more time fighting the evils of the galaxy than developing their interpersonal skills. Remember, you're paying your dues; your master might not really be that clueless, just testing you. Then again, he (or she) just might be that thickskulled.
Eventually, your master will sponsor you to become a Jedi Knight, and you will go through a ceremony to, among other things, build the lightsaber you will carry for the rest of your life. Congratulations! You're now a Jedi, protector of good, defender of the balance yadda yadda yadda. Just remember, strangling people is of the Dark Side, so don't get too happy.
Okay, so you're not in the Republic. Are you in the Empire? (Look for Stormtroopers. They should be everywhere.)
Are you really, really sure this is a good idea? If you become a Jedi, you've just painted a big red target on your butt. Yeah, the Stormtroopers are wimps, but Darth Vader is a really unfriendly guy.
Anyway, you're a daring kind of person, so you need to go find a Jedi - Knight or Master, it doesn't matter at this point - and pester him/her until he/she trains you. This is really going to vary depending on the master; Obi-Wan Kenobi seems amenable to anything that will get him out of the house, but Yoda will steal your stuff and beat you with a stick.
From here, just follow your master's instructions. One he/she decides you're a Jedi Knight, you are, seeing as there's nobody who's particularly inclined to argue. Congratulations, now you get to be killed by Darth Vader.
Are you living in the New Republic? (Look for arguing Senators and children whose last name is Solo.)
You're in luck! Sort of. Hitchhike your way to Yavin IV (I suggest hitching a ride on whatever attack ship is headed in that direction this week) and simply pester Luke Skywalker until he lets you join. If he's not about, Tionne, plot piece that she is, will probably be happy to sing you some droning song about the epic of Bill the Jedi from the 3125 years before common era and how he filled out how paperwork to become a Jedi.
The cirriculum here is much simpler; generally, it's two parts. "Don't Die" and "Save The Galaxy." While the first part is easy (avoid the Unspeakable Horrors and any stray blaster fire, and you'll do fine), you need to make sure you have some hand in saving the galaxy. While there's always the direct route of actually saving the galaxy yourself, like Corren Horn, this directly conflicts with the first goal of not dying. (Alternately, you could threaten the galaxy yourself, then save it; just ask Kyp Durron, but this isn't recommended.) Instead, when someone else saves the galaxy, make sure you're hanging around somewhere safe. Preferably the same planet, just a different continent. The Force links all living things, so you were helping, right?
Anyway, now that you're qualified, just go through Luke's interminable ceremony and build yourself a lightsaber. Preferably one that won't burn your fingers off. Congratulations, you're now qualified to bore the Senate, lead apprentices to their doom, and make fun of Luke.
There. Now you know how to become a Jedi.
"But this good guy thing sucks. I wanna have the powers of the Force, but I don't want to put up with any of this good guy crap."
Then what you want to be is a Sith. This is easy, as long as you don't have a morbid fear of dying. First, count the number of Sith in the galaxy. If there's a whole bunch of them, just go up to one and hope he/she doesn't kill you. If he/she doesn't, you're set. If there's only two, bump one off and bug the other until they relent (or kill you). If there aren't any Sith, just go hunting for a library of Sith lore, then fry a Jedi or two.
Sources: Lots and lots and lots of bad Star Wars novels. I hope Kevin J. Anderson burns for his sins.