A thesis is a statement that is often a single sentence found at the end of your introductory paragraph in an essay. It typically summarizes what the rest of the following paragraphs will contain.
For example:
Paragraph 1: Introduction
Paragraph 2: A
Paragraph 3: B
Paragraph 4: C
Paragraph 5: Conclusion
If the above was your essay format, your introduction paragraph would likely look something like this:
{Introduction paragraph} The zoo is very important for our societal wellbeing, it allows us to A, provides us with B, and helps our economy with C. A, B, and C would then be explained in paragraphs 2, 3, and 4, as the outline suggests.
After this introduction paragraph and thesis statement, you proceed with the rest of your essay.