The first and second rows are reserved for the immediate family, parents, siblings and grandparents. If your parents are divorced, the parent you lived with sits in the first row with your stepparent. Your other parent and their family sit in the second row. The third through six rows are usually reserved for uncles, aunts, cousins, godparents and any other special guests.
In a Christian ceremony, the mother of the bride is always seated last. (The groom's mother is seated right before the bride's mother). Once the bride's mother has taken her seat, the ceremony begins.
Traditionally, in a Christian ceremony the bride's family is seated on the left (when the guests enter from the back). The groom's family is seated on the right. For a Jewish wedding, it is reversed. If either the bride or the groom have a much larger family than the other, it is a good idea to seat the guests on either side so it looks balanced.
If it is a formal wedding, the ushers will escort the female guests to their seats while her male escort follows them. If your ceremony is informal, the ushers can greet your guests at the door and say, "Please follow me". For every 50 guests, you should have one usher.