There are a few qualities that make for a good topic sentence:
Brevity: Long, rambling sentences can be confusing. Don’t pack your topic sentence too full of details. That’s what the rest of the paragraph is for.
Clarity: Likewise, don’t beat around the bush. Say exactly what you want to say. Try not to engage in wordplay and don’t speak in vague terms.
Precision: Don’t be too broad when introducing the topic that you’re going to discuss. Not only is that a bland approach, it is also unhelpful to readers.
On the other hand, there are a few things that make for a bad topic sentence. Be sure that you aren’t using these kinds of sentences to introduce your paragraphs, as they are unhelpful to readers and do little to help you get your point across.
Don’t use facts as topic sentences. Remember, you are using the topic sentence to introduce a point you are trying to make, or your opinion.
Don’t just talk about the “what” – talk about the “why” as well. That is, don’t just think about the effect, but it’s cause.
Don’t say “I am going to tell you…” or I am going to speak about…” when introducing a topic.