According to the Code of Jewish Law (the "Shulchan Aruch"), there are three requirements for a valid conversion:

1) Mikveh - All converts must immerse in the Mikveh - a ritual bath linked to a reservoir of rain water.

2) Milah - Male converts must undergo circumcision by a qualified "Mohel." If he was previously circumcised by a doctor, he then undergoes a ritual called "hatafas dam."

3) Mitzvot - This is the clincher. The convert must believe in God and the divinity of the Torah, as well as accept upon himself to observe all 613 mitzvot (commandments) of the Torah. This includes observance of Shabbat, Kashrut, etc. -- as detailed in the Code of Jewish Law, the authoritative source for Jewish observance. This means that a motor vehicle is not used on Shabbat, that cheese is eaten only with kosher supervision, that a woman uses the mikveh every month, that hands are ritually washed before every bread meal, that the status of a Kohen is preserved, and much much more.

All of the above must be done before a court of three Jewish men who themselves believe in God, accept the divinity of the Torah, and observe the mitzvot. In the case of someone who denies fundamental principles of Jewish belief (such as, the word for word divinity of the Torah), or offers to perform the conversion without requiring full mitzvah observance, the conversion would be invalid according to the Code of Jewish Law.

The candidate must be emotionally ready to fulfill all the 613 mitzvot, as well as have learned enough of their details to be able to fulfill them properly. 

There are two excellent books which are helpful for conversion:
- "To Be A Jew" by Chaim Halevi Donin
- "Becoming a Jew" by Maurice Lamm

Also recommended are two real-life accounts of non-Jews who converted to Judaism:
- "Migrant Soul" by Avi Shafran
- "The Bamboo Cradle" by Avraham Schwartzbaum