Firstly, the normal concept of Free Will means you have some sort of control over your actions. (Some people redefine it otherwise, and if they want to use weird definitions like that, it doesn't help people who favour the usual conception of Free Will.) This sort of Free Will is, I believe, logically impossible. It's possible that it's merely a physical contradiction, but I see it as a stronger problem than that. Anyway, here's the problem:
Every event x1 (a rainstorm, a quantum fluctuation, a supposedly free choice) is either caused by a prior event (or events), or not caused by anything.
If an event x1 is not caused by any prior event then it was not caused one's Free Will and thus is not a free choice.
If event x1 was caused by a prior event x2 (or prior events x2, y2, z2, etc.) then that event x2 (and y2, z2) are themselves either caused by prior event(s) x3 (y3, etc.). This chain of events can be followed back until one reaches an uncaused event or an event from before one's birth.
Thus, any supposedly free choices one makes is the sole consequence of things which you had no control over, or is itself an event which your Free Will did not cause (and neither did anything else).