By the time you are a senior in high school, you should be able to write an essay according to whatever format is asked of you (. persuasive, argumentative, narrative, compare/contrast, etc.). With that being said, the thesis statement can be, and usually is, included in the first paragraph. The introductory paragraph describes the issues that you will be discussing while also presenting your strong opinion in one sentence and written in 3rd person (Don't use "I" or "you"). Strong theses generally state that an issue should be addressed in a certain way without identifying yourself with a personal pronoun. Topic sentences are the main sentences presented in each following paragraph after the introduction. They can give you a focus of what to write about in each paragraph so you don't go off topic and discuss another issue at the same time. (This can confuse a reader.) Topic sentences guide the discussion of each paragraph, basically. For example, if the second paragraph will be about elephants, then don't start talking about tigers in that paragraph; save tigers for the third paragraph, instead. Remember, too, that the concluding paragraph can touch on all of the main points again as long as you drive home the main reason why you discussed them in the first place.