There are a few different ways to become a werewolf.
Becoming a werewolf, though, is a decision that must be thought out well ahead of time, if at all possible. Some methods of becoming a werewolf are more dangerous than others, and life after transformation may not be what you expect.
If you haven't yet, please read the primer on Becoming A Werewolf. It will give you all of the information you need to make an educated decision on whether or not lycanthropy is right for you.
That said, let's get to it. There are three ways to become a werewolf:

1. Be born to parents with werewolf genetics


2. Be bitten by a werewolf


3. Be "cursed"

Growing Up Werewolf

I suppose it's a bit of a misnomer putting this in "Ways to Become a Werewolf", since if you aren't already born this way you can't do anything about it. Technically, though, it is a valid option, so I guess it stays.
In order to be born with the genetic ability to become a werewolf (known formally as "Genetic Lycanthropy"), both parents have to have the werewolf gene to pass on to their child, though neither parent necessarily has to be a werewolf themselves.


For a better understanding of how this works, I've prepared a Primer on Werewolf Genetics that will give you a detailed, scientific explanation of how exactly a child can be born a werewolf.
Those born with genetic lycanthropy are not werewolves right out of the womb. A human with the genetic structure will not begin the werewolf transformation until sometime in late puberty. This can be greatly beneficial to the child, as it has a better chance of survival in society in its human form. Of course, if the human doesn't know they will become a werewolf, this can understandably come as quite a shock.
When genetic werewolves begin to transform, they do so in a very similar way to bitten werewolves or cursed werewolves. They begin as more man beast, then over time reach the form of a pure wolf after transformation.

Being Bitten

A human that was born without genetic lycanthropy isn't yet out of the werewolf game. There are still two other ways to become a werewolf, the most common being through a werewolf bite.
Much like vampires, werewolves carry a bacteria in their bodies that is toxic to humans. It is this bacteria that can cause the human body to be capable of werewolf transformation.
Unlike vampires, werewolves don't carry a significant amount of this bacteria in their bloodstream. At least not the strains that can infect a human. Werewolves almost exclusively carry the infectious bacteria in their saliva. Thus, when a human is bitten by a werewolf, there is a good chance that the saliva will enter the human's blood stream where the bacteria can do its work.
If you are bitten and don't want to become a werewolf, it is best to begin an antibiotic treatment as soon as possible. Success rates are not great with antibiotics, but they are the only known way to possibly kill the bacteria.
Following a bite, the bacteria strain will begin duplicating itself immediately in the human bloodstream. The process takes roughly six to nine weeks to fully integrate the human cells. During this time the human may experience insomnia, headaches, and flu-like symptoms, but will not show any external signs of werewolfism.
Once the bacteria has fully integrated with the cells, the human will become a werewolf, and as such will experience transformation each night from that point forward.

The Werewolf Curse

Of the three ways to become a werewolf, the curse is by far the most rare. In fact, we know very little about it other than that it exists.

The first werewolf was created by this curse, so we know that the original deity that created the curse has the ability to give it to others.

Some later tribal legends suggest that the original werewolf, Matchitehew, was able to bestow this curse onto others, and though it is clear that he did so without biting people, it is not totally clear if he could "cast" the werewolf curse onto them directly.
There are myths of black magic spells to transform people into werewolves, but again, the only one I have seen that works with the rest of werewolf mythology is the story of the original curse.