Everything is predetermined, because atoms (and until proven to the contrary all other matter larger or smaller) behave according to a set of rules, bouncing off one another in a way that would be predictable given enough knowledge of what drives them. Our consciousness is a reflection of the interaction of our brain cells, [etc] which is entirely the result of how atoms combine and interact with one another. For all I know, Quantum Mechanics may be random, but that doesn't change the fact that we are at it's mercy.
This is from an agnostic's point of view, and therefore with the assumption that there's no such thing as a soul. Although even if there were such a thing as a soul, I'd still feel the same way although the explanation is a bit harder to vulgarize. For more on that, I recommend Schopenhauer's "On the Freedom of the Will," which is outlined on Wikipedia, an essay who's conclusion matches my own, although with many differences in some fundamental areas (He claims virtue cannot be taught, whereas I think it's more likely to be taught than innate).
But since no one is going to actually read it, I'll have to do my best:
People can do as they please, but they cannot choose what they want. People make decisions based on what they happen to desire - their motivation, and on how they reason to achieve those things - judgement. Both things can be either innate and taught, for example, your desire to eat is innate, your love of video games is "learnt" in this case. Everyone always does something for a reason, no matter how insignificant or baffling to the observer, and given your preferences and the logic you used you can be expected to always come to the same conclusion and make the same decision if someone else were to rewind and watch you make a decision again.
Now for a few examples people like to bring up as contradiction:
Say you wake up and have a choice between two cereals. Each seems equally enticing and so you have to think about which one to take. It may look like it can go either way, but in reality you were always going to go for the Cocoa Puffs because at the last second you felt a slight craving for chocolate, stronger than your current taste for honey & nut. What else could you have done? Not have a craving for chocolate? What if you realize that you might have been better off with Raisin Bran? Doesn't that contradict it? Well not quite, because I would conjecture that at that time your thoughts turned towards health, and your need for health trumped your need for indulgence this time. Finally, one might say that thinking about it was a decision, and that was just someone making the right choice, and the cycle continues with the obvious (I hope by now) "But did you decide to have good judgement, any more than you decided to be good or bad at algebra?" That's why Schopenhauer says (paraphrase) "You can do what you want, but you can't choose what you want."
A lot of people don't like this way of thinking because it's depressing, but I disagree. For one thing, if you ask me it's the lesser of two evils. The alternative to everything happening necessarily as a consequence of something else is that decisions come at random, effectively a coin-toss each time. But if everything is a coin-toss, how can you judge someone as being good or bad?
Another positive is that I can forgive people for being what I consider bigots. I mean, if no one taught them otherwise, they couldn't possibly have been un-bigots unless they were just hard-wired from birth to be deep thinkers and came to the conclusion on their own. And it would be the height of arrogance to think that born into their shoes that one could do better.
Religious people might have a problem in that this entails that you were doomed to Hell or Heaven from the start (well, Calvinists won't mind), and since this is religious I can't touch it.
And finally, there are people who dislike this philosophy for sentimental reasons. Well, tough luck, but wanting really hard to be fully autonomous won't make it true. But I'm a glass-half-full guy, I can take it same as a roller-coaster ride. Sorry OP, I never meant for this to be an essay, but I had to defend my point.
