Once your character has reached level 15 and become sufficiently aligned to the light side (75-100) or dark side (0-25) of the Force (signalled by a cutscene of Visas and her master), speaking to Kreia will open the following dialog:
You can now choose to become one of three Prestige classes.
The time to hide your presence is coming to a close, and you will need to increase your training.
You are ready. Your training must increase, and there are higher mysteries you must learn. But only you must know the path you will take, I cannot choose it for you.
There are Prestige classes analogous to each Jedi class, depending on your alignment.
However, your character's existing class does not restrict your choice. For example, a Jedi Consular can become a Jedi Weapon Master if light side, or a Sith Marauder if dark side.
Jedi
Is it battle that stirs you, to meet an enemy blade upon blade? Such is the way of the greatest of Jedi warriors, the weapon masters.
Or perhaps it is investigating the mysteries of the galaxy, seeking out injustice and harm, and bringing it into the light?
Such is the way of the Jedi Watchmen in the time of Ulic Qel-Droma… and Exar Kun.
Or perhaps your way lies upon the ancient mysteries… and to teach others the ways of the Force, as I have, the way of the Jedi Masters.
Is it battle that stirs you, to meet an enemy blade upon blade? Such is the way of the greatest of the Sith Marauders, who seek strength in war… and their rage.
Or is it the ways of darkness and shadow that you seek, the power that comes from striking unseen and sensing weaknesses in others… the ways of the assassins of the Sith.
Or perhaps your way lies with the Sith… and the heart of their darkness. To recognize what strength is, and bring that strength to those that follow you… and those that defy you. The way of a Sith Lord.
You can also choose to return later, when you are ready to learn the higher mysteries of the Force (in fact, you do not have to become a Prestige class at all). Otherwise, you will gain your first Prestige class level upon the next level up.
It is not some great test you require to be what you strive to be – it is only your decision to find that path that matters.
From here on, you guide your destiny. But in order to take the next steps, you must face your past and put it to rest.