What the heck is a thesis, anyway?
Here's the problem with the dictionary definition: it doesn't tell you how to write one. So, the real question isn't "What?" it's "How?"
A thesis is an opinion that is arguable. In this case, it's your opinion. (That's right: people want to know your opinion!) The rest of your essay will focus on convincing your reader that your opinion is the right one, or at least a relevant one with evidence to back it up.
Here's how to pick one: To make sure you have a thesis that will work, start by collecting evidence (examples or samples or quotes or actions) that back up your claim. Choose a couple of potential theses that interest you, and see if you can find three pieces of evidence to back each one up. If you can, you have a winner!
Here's the hard part: a fact is not a thesis. If your emotions tell you that something you read is right, wrong, crazy, or awesome ... well, that's not enough. It might be a great place to start, but you need evidence to use it as your thesis.
It has to be arguable: If almost everyone who reads the novel or article will agree with you that Joe is a good guy or that stealing is a bad idea then there's no one to argue with. Your thesis might work but it's not a very strong one.
Your thesis is the backbone of your essay; choosing a strong one will make your writing task much easier. Please do let me know if I can help you with this!