Writing an essay can be a difficult enough task as it is; writing it a language that is your first language is even harder.
Paragraph One: The Introduction
This first paragraph, made up of 3-5 sentences, has two purposes: grabbing the reader's attention, and providing the main point (thesis) of the whole essay.
To get the reader's attention, your first few sentences are key. Use descriptive words, an anecdote, a striking question or an interesting fact related to your topic to draw the reader in.
To state your main point, your last sentence in the first paragraph is key. Your first few sentences of the introduction basically introduce the topic and grab the reader's attention. The last sentence of the introduction tells the reader what you think about the assigned topic and lists the points that you're going to write about in the essay.
Here's an example of a good introductory paragraph given the topic, "Do you think teenagers should have jobs while they are still students?":
I've worked ever since I was twelve. As a teenager, I cleaned houses for my family members, made banana splits at an ice cream parlor, and waited tables at various restaurants. I did it all while carrying a pretty good grade point average in school, too! I definitely believe that teenagers should have jobs while they are still students because a job teaches discipline, earns them cash for school, and keeps them out of trouble.