Video from Crossexamined.org
Lets make a distinction between determinism and fatalism. Calvinists believe in a form of determinism : God has determined every single event. At each moment there is only one possible future: the future God has determined. But lets also clarify what we mean by determined. God can see the beginning from the end. As God the Father exists outside of time looking at the expanse of creation in one flash from beginning to end he knows your decisions even if you have not made them yet. In that sense your life in eternity is pre-determined.
This is not to be confused with fatalism which is the view that our choices don’t affect the future. Many Christians think of God’s providence in the wrong way: “If God has determined every future event, then my choices don’t affect the future.”
As you and I are co-regents with God in his Kingdom what we pray and do has impact or what we fail to do (omission and commission). God not only ordains the ends, he also ordains means which includes the destination and the entire journey of how we get there. When God determined/observed/knew ahead of time that Christ would die on the cross, he also determined the means by which he was to be killed, the means by which he was delivered to the authorities, and the means by which he was betrayed.
God governs all events in his universe—including the “small” ones leading up to the “big” ones. This is often called meticulous providence.
What happens in the future, then, does depend on what we do and pray in the present. So don't discount your prayers made faithfully everyday as they are all part of the great weave and tapestry of God's creation.