How do you become a Jew?

When God called Abraham to leave his homeland and family He gave him a wonderful promise: “I will bless those who bless you, and whoever curses you I will curse; and in you all the families of the earth shall be blessed “(Genesis 12:3).

Years later, after the birth of Ishmael, God made a covenant with Abraham and promised to make him into many nations (Genesis 17: 2.5-6). God gave as condition that all males must be circumcised.

When the Israelites left Egypt, a large group of people from different backgrounds went with them (Exodus 12:38). The stranger who wanted to join with the Passover feast, had to be circumcised. He was then considered a native of the nation of Israel (Exodus 12:48).

During their stay in the wilderness, the Israelites were given all the laws they needed to live in harmony with each other and all the instructions concerning the worship of God. These all applied to the stranger who had become an Israelite as well (Numbers 15:15-16).
Still, the main condition to become part of God’s people was circumcision.

Even today those who want to become a Jew must be circumcised. If the man is already circumcised before, a special ritual called hatafat dam brit is performed, where a drop of blood is used to symbolically perform the circumcision.*

* Sources:
http://www.myjewishlearning.com/life/Life_Events/Conversion/Conversion_Process/Circumcision.shtml
http://www.beingjewish.com/conversion/becomingjewish.html