Information from sensors plus mood state (local chemicals produced by DNA out of what was eaten/drunk recently - they change neuron threshold value, amplitude of signals, etc) plus/minus some quantum fluctuations (due to small amount of some chemicls produced: it may be 15 protein molecules in a neuron today and 17 tomorrow due to quantum fluctuations, but at threshold value of 16 molecules per cell it may make a difference).

So, frankly, I do not see a source nor a room for a "free will" (not to say about Santas or gods switching cells on and off).

What you call a free will is just an interplay of several competing physical factors at the time of decision.