Why does Tituba confess so readily? What does her confession initiate?

To answer this question best, it helps to put yourself in Tituba's situation.  She holds a very inferior position in her society; she is a slave, and her worth is not counted for much.  To start off, she is asked by a prominent member of the town, Mrs. Putnam, to conjure spirits for her, to find out if any evil spirit had killed off Putnam's babies.  Mrs. Putnam is a friend of Tituba's master, Parris.  Also, Abigail, Parris's niece, asks her to make a charm.  So, you have two white women that are close to her master coming to her and begging her to do these things.  Then, she gets caught at doing them.  She could be punished severely--sold, whipped, taken away, any number of things.

THEN, to top it all off, she has Abigial, who "begged [her] to make [a] charm," blaming the entire scenario--the dancing, the pot with the frog--on Tituba.  Abby cracks, screeching out, "Tituba!  Tituba made me do it!"  Abby was afraid of getting in trouble for the dancing and spell-casting, so she blames Tituba for doing it.  Now, everyone in the room turns on Tituba.  Imagine an entire roomful of people bearing down on you, one being your master, and then Reverend Hale, putting tons of pressure on you.  Parris threatens a good whipping if Tituba doesn't confess to witchcraft, and Hale is in her face, demanding that she confess.  Then, his key words are that if she does confess, it will be much easier for her; she won't be punished as badly.

So, look at her options.  She can NOT confess and deny the entire thing, and end up being accused of being a witch and being hanged and whipped, OR, she can confess and blame someone else, and not be whipped or hanged.  She can pass all possible repercussions on to some other poor soul.  So, she chooses the latter option, confessing so that she can get out of punishment.

A soon as she does confess, she is praised by Hale as an angel sent from God to root out evil.  She is treated kindly, with love and affection.  Abby, one smart cookie, sees this turnaround and thinks to herself, "Hey, I think I'll try that," and starts "confessing" and blaming others for bewitching her.  The other girls follow suit.  So Tituba's confession initiates all of the accusations that follow that lead to the witch trials.

I hope that those thoughts help; good luck!