The universe is deterministic. The state it is in now determines the state it will be in in the future. As such, the cause of the effects that will make your brain think and your body move have been pre-determined since the beginning of time. It is our consciousness that gives us the delusion of free will.
At least, that is what some scientists are stating.
In addition, before you think of anything, your subconscious brain has already started processing data, likely informing your conscious of what 'you' are thinking. If your subconscious knows what you will do before you do, and possibly tells you what to do, where is your free will?
Really, Free will is one of many illusions we live with and become so comfortable with that we usually don't think about it, much like our sense of sight. For all intents and purposes, we are a brain in a vat, however instead of a supercomputer giving us our input, our senses are, and we create illusions based off what they tell us.