Your answers lies in the purpose of the essay and that is persuasion.
Ask yourself "If I was listening to this talk how would I be persuaded?" How could I persuade myself if I needed to?
Each outline and planning topic sentences and indentifying supporting points will vary depending on the purpose of the essay.
You will need to first be firm on what you want to persuade your audience. You can't be wishy-washing here. That won't help. You need to believe in your subject.
Each topic of persuasion will have areas you want to cover to prove your idea.
Say you want your audience to understand the importance of planned grocery shopping since this saves money and saves wasted food. Ask yourself
Why, When, Where, How? These will be your main points in the outline.
Your supporting points will be 1. why because A....B...,etc.
The topic sentences will depend on your point but remember they will need to be specific to your point, not general. You topic sentences can have humor, (depending on the subject)but don't overdo it.
Also in your supporting points be sure your information is current and correct with references you can prove and are credible. Nothing ruins a persuasive essay faster than sloppy research. Depending on your audience
you may find your audience asking "Where did he or she get that information? That doesn't sound valid." Then you will lose them.
Your topic sentence should be very powerful in a persuasive essay. If you need ideas ask your teacher for feedback or brainstorm with friends.
Does this help?