Research your topic to get a good overview of the subject. For example, if your topic is bears, you could research bear eating habits and physical characteristics. You don't need to know everything about bears that has ever been discovered, but get a good general idea of the topic.
Form an opinion about bears. It is best at this stage to choose a wider opinion than a narrower one, as you will narrow it down for your thesis statement. For example, you could decide "Bears are scary."
Research your new opinion ("Bears are scary") thoroughly, going into more detail than you did your overarching topic (bears). Study specifically why bears are scary. What features, habits and traits do bears have that make them scary to you? You do not need to do deeper research into subjects such as hibernation or the characteristics of a bear's fur.
Choose the aspect of your thesis that is, in your opinion, most appropriate to your thesis that bears are scary. Consider if you can marshal the arguments to properly support your thesis. Consider also if the thesis is too narrow or too broad. Something along the lines of "Bears should not be approached in the wild due to their extreme strength and reserved personalities" is narrow enough to provide a focus (strength and personality) while not being so narrow that the essay is one paragraph long.