A weak thesis restates the topic. A strong one makes a claim someone could disagree with.
It should be arguable
"Social media affects teenagers" is not a thesis — nobody disagrees with it. "Social media use correlates with lower attention spans, but the causal link is weaker than most coverage suggests" is a thesis, because a reasonable person could push back on it.
It should be specific
Name the mechanism, the population, or the time period your paper actually covers. Specificity is what lets a reader (and a grader) check whether you proved what you claimed.
It should preview your structure
A good thesis often hints at the shape of the argument to come, so the reader knows roughly what is coming in the body paragraphs.