Question about Predestination

I was talking to my aunt who is a Roman Catholic and she tried to explain to me how God gave man the gift of free-will, but I was confused. Maybe you can help me? She described her God as an all knowing, all powerful entity, among other things. So here goes:

If God is all-knowing, then he will know what I will do before I do it. Based on the fact that God's knowledge can't be false, how has God given me the ability to change my destiny, if the path I am going to take is already written?