No father-figure to walk you down the aisle? Whether due to divorce, death, or you simply don't have a relationship with your father, there are plenty of other options available.
If you would prefer to be escorted, you can ask any male relative you feel closest to, even if that relative is younger, such as a younger sibling. No male relatives? Try a close family friend. If your father has passed away, perhaps ask his best friend. Is one of your closest friends a man? Many couples find it awkward to have men standing with the bridesmaids, if both you and your fiancé are comfortable with that friend escorting you; it is a great way to honor that relationship.
Also, remember that it is perfectly fine to be escorted down the aisle by your mother or grandmother in these situations.
For many brides with awkward family situations, or also due to personal preference, they choose to walk down the aisle alone. This type of processional is happening with increasing frequency, and you needn't feel self-conscious or insecure about it. This is your time to shine!