You (and everyone else) certainly can choose to do what they want at any time, but in the end they end up doing whatever it is they do because the chemical reactions in their brain compele them to do so. Determinism isn't about people not being able to think about decisions, but that their thoughts are all driven by chemical reactions, which were caused by other things in the world, which had their own causes, etc. Basically, in order for determinism to be incorrect, there needs to be a way that people generate thoughts/decisions that is not based on a reaction in the brain. I'm not the best at explaining either so i'm sure i'm doing a shitty job of it but hopefully you get my point =/