If you're writing a single paragraph essay, be it for a class like ours or an answer to an essay question on a test, you have two basic designs.

This simple box represents our single 'graph. The black bar is the first sentence of the 'graph. If it is the main sentence in this collection of sentences, it is the
topic sentence. The topic sentence is your
thesis for the paragraph, the reason you're writing what you are.
For example, one of the one-paragraph assignments I used to give my Comp I students was the following:
Tell me the one thing you'd want to have with you if you were stranded on a desert island. Assume food and water are available in sufficient quantity though not necessarily easily obtained. This isn't Eden. You cannot take another person or a pet with you. You are going to be utterly alone for a very long time.
I used this assignment almost every term until that Tom Hanks' movie came out. After that, all anyone ever wanted to do was some variation on Wilson the volleyball. One early attempt I still remember went something like, "I would take a radio capable of letting me listen into the world I was no longer a part of because without such a connection I would go mad."
After all of these years, that's an approximation, but it was a very moving little essay this fella crafted when I was still working for Kirkwood. The point here is that when you begin your paragraph with your topic sentence then everything in that 'graph needs to be written in such a way that it supports your thesis.
If we reverse this design we get a box like the one that follows:

This paragraph is more or less the same as the first except everything you write must lead up to supporting the statement, your topic sentence, that is going to conclude the 'graph.
The first design, or method, of putting the topic sentence at the very front of the 'graph is the most often used, especially in essay tests. You don't want your instructor being uncertain of your intent or meaning. Even if you are trying to bluff your way through something, you need to start strong with a good, solid topic sentence--with a thesis that sounds as if you're on top of things.
If this second design is invoked, it is often used when your thesis is going to be controversial to your reader--or, at the least, unusual.
I don't have an especially good example to give you here, but let's say you're taking a health occupations class and have been studying the spine and its problems. The instructor gives you a test asking for some written answer intended to summarize why so many of us are afflicted with back pain. You have an idea of what the instructor's after, but you've been doing some research on your own and have come up with your own thesis about back complaints.
You want to say this:
Though there are many reasons for back problems and spinal pain that are unique to individuals or groups of individuals, the primary reason so many humans suffer from back ailments is that our ancestors came from a planet with less gravity and a more oxygen-rich atmosphere. Friend, you better have a boatload of reasons preceding that statement, and you better stick that thesis way-y-y-y-y at the end of your paragraph.
Keep these methods or designs in mind as you do the shorter assignments, our general assignments, for the rest of our class. Even if it's a journal entry, give it a little more thought and structure your words and sentences in one of these ways.