It does vary, but in a traditional "American" wedding (although you can do whatever you darn well please), the bride is escorted down the aisle with her father on her right. The bridesmaids will already be at the front lined up on the left (if you're looking at the altar from the congregation), and the groom and groomsmen on the right. The father of the bride will stand there until the officiant asks "who gives this bride", at which point he generally answers "her mother and I do", after which he takes his seat on the front row.