A thesis is a claim someone could disagree with
The most common weak thesis is a fact in disguise. 'Pollution is a problem in cities' is something nobody argues with, so an essay defending it has nothing to do. A thesis must take a position that a reasonable person could oppose.
The test is simple: can you write a sentence that contradicts your thesis and still sound sensible? If not, your thesis is a topic, not a claim.
A structure you can fill in
Use this shape until you no longer need it: 'Although [the strongest opposing view], [your claim], because [reason one] and [reason two].' The concession makes your position look considered, and the two reasons quietly become your body paragraphs.
Example: 'Although school uniforms reduce visible differences in wealth, schools should not require them, because they do little to change how students actually judge each other and they shift costs onto families who can least afford them.'
Before and after
Watch how each rewrite gains a position and loses vagueness.
- Before: 'This essay is about social media.' After: 'Schools should teach social media literacy as a required subject, because students already spend hours in an environment nobody has taught them to read critically.'
- Before: 'Romeo and Juliet is a tragedy about love.' After: 'Romeo and Juliet is less a tragedy of love than a tragedy of haste, since almost every fatal turn follows a decision made within a single day.'
- Before: 'Recycling is good for the environment.' After: 'Household recycling deserves less attention than it receives, because the majority of packaging waste is decided by manufacturers long before it reaches a household bin.'
Where it goes and how long it should be
In a standard school essay the thesis is the last sentence or two of the introduction, after enough context that the reader understands the question. One sentence is usually enough; two is fine when the concession is long. Three sentences almost always means you have merged your thesis with your first body paragraph.
Revise it after you finish writing
Your essay will drift as you write it, and that drift is often an improvement. Reread your conclusion, notice what you actually argued, and rewrite the thesis to match. Graders reward an essay whose opening promise matches its ending far more than one that follows an outline it outgrew.