Bemidbar (18:15-16): "You shall redeem the firstborn of man; the redemption [shall be performed] from the age of a month, according to the valuation, five shekels of silver."
The redemption starts in the middle of the festive meal. All begin to eat, and during the meal the baby is brought to the father and the Kohein. Some have the custom to decorate the baby with jewels and sugar cubes (the sugar cubes are given to the children afterwards, the jewels returned to the owners).
Many have the custom to distribute cloves of garlic during the meal. The reason for this seems to be that eating from the food distributed at the Redemption Meal of a firstborn is considered very praiseworthy, and garlic was very cheap. Besides which, one head of garlic can be divided up into many cloves, so it is possible to give many people one small piece very cheaply, and everyone has a small share in the Mitzvah Feast. There are also Kabbalistic reasons as well, apparently.
This festive meal should not be treated as a mundane meal, and should include discussions of Torah. It is customary to invite Torah Scholars to enjoy the meal, and ask them to speak words of Torah and inspiration to those in attendance.
In Egypt, the night before the Israelites left Egypt, the Firstborn children of the Israelites were saved from death, as all the firstborn of Egypt were dying. While that was happening, the Israelites were sitting at the very first Passover Seder, and eating the Passover Lamb Sacrifice. They sat, spoke Torah, and sang praises to HaShem for His kindness to the Children of Israel. As you eat at the Festive Meal Celebrating the Redeeming of your (or your friend's) Firstborn Son, remember to thank and praise HaShem, as we did when our firstborn children were saved back then.
The Medrash Hagadol states that one of the taamim for the mitzvah of Pidyon Haben is that it is to provide a tikkun and a kapporah to Klal Yisrael for mechiras Yosef by his brothers.
Our sages say that partaking of the pidyon haben meal is the equivalent of fasting for 84 days! That's why it's a tradition to serve packets of sugar and garlic—food that goes a long way for a long time.
