What happens during a Bar Mitzvah?

The Bar Mitzvah and its particularities are dependent on the Movement of Judaism to which the family of the Bar Mitzvah subscribes and whether or not the service chosen is a Friday-Night Service (Kabalat Shabbat), a Saturday-Morning Service (Shacharit shel Shabbat), or a Weekday (Shacharit Shevoua). Reform Synagogues tend to do a Friday-Night Service whereas Conservative and Orthodox Synagogues tend to do a Saturday-Morning Service. Weekday Services are usually from Conservative Synagogues that have too many Bar Mitzvot in a given year and need to spread them out. Check the Synagogue in question to see the length of these services, the prayers they contain, general movements and customs, and the Hebrew to English ratio.

The Normal Service (of the equivalent type) changes in a few noticeable ways during a Bar Mitzvah. Most noticeably, on Saturdays or Weekdays, the Bar Mitzvah (the thirteen-year-old boy) is required to read a few lines from the Torah Scroll with the proper cantilation technique. (Normally this would be done by a Ba'al Koreh or a Congregant.) On Saturdays, the Bar Mitzvah is also typically tasked with reading the Haftarah, a section of the remainder of the Bible which has a message parallel to the Torah reading for that day. (This would also normally be done by a Congregant.) Finally, the Bar Mitzvah always gives a Dvar Torah, or a sermon on the lessons to be gained from the Torah or Haftorah Portion.
Usually the young man will read a portion from the Torah or the prophets. A celebratory meal is also held.
At a Bar or Bat Mitzvah, a 13 years old boy or 12 years old girl reads from the Torah in front of the congregation for the first time. A Bar or Bat Mitzvah is when the boy or girl formally takes on the religious responsibilities of an adult. Most people have a huge party once Shabbat is over.

Bar or Bat Mitzvahs are usually on Friday nights or Saturday mornings, during Shabbat, the Jewish sabbath. Sometimes you have your Bar or Bat Mitzvah with a partner.
the boy reads from the Torah and final becomes adult