Traditionally the bride and groom sit in the middle of the table with the parents of the bride and groom on either side. The best man, maid of honor, bridesmaids, and groomsmen fill out the rest of the table. If the couple decides not to have a bridal party, the siblings and best friends who would have made up said party sit at the main table.
Table considerations also include step parents and divorced parents, as sometimes careful seat arrangements are necessary. How formal or not you wish to be about your wedding is of course up to you–you may want to forgo the head table entirely and sit among your people! Remember, it is your wedding so feel free to do whatever you want. Just keep in mind that if you choose to have a head table, it is for you, your groom, and the people closest to you. The head table also makes it easy to call attention to those giving speeches, such as the maid of honor and the best man.