The very first thing that you want to do is pick a topic that you feel passionately about. Your audience will be able to tell right away if you are excited about the thing that you are trying to persuade them about or not. I don't know what your teacher is requiring for a topic. I teach a speech class, and I have my students do a persuasive speech. The topic is wide open, but I usually tell students to do a persuasive sales speech. Pick a product and sell the audience on why that product is great. Doing that will allow you to pick something that you like. Pick a restaurant or drink of some kind. Pick a video game console or car or car manufacturer. It doesn't matter as long as you have personal experience with the product and like it.
Multimodal typically refers to different types of learners. In general, there are three categories: visual, auditory, and kinesthetic (touching/doing).
Auditory is simple. You are giving a speech. Most of your information is going to be conveyed by your voice. What you say and how you say it covers auditory.
Visual can be done in a variety of ways. First, use gestures. Please. Your audience does not want to listen to a speech and watch a statue. If using gestures is too nerve-wracking because it makes you nervous to have people looking at you, then find a way to divert their visual attention. Use visual aids. Let's say that you are doing a speech about the candy company Mars. You can have a poster on an easel with pictures of that company's products. Or use a powerpoint and a projector. Or better yet, have a table next to you with a selection of Mars candies. Or use a combination of those methods. You can have written information on the powerpoint that parallels your speech and the display table to further highlight the product.
Kinesthetic might be your most difficult, but that can be accomplished by having your audience physically participate at one point in the speech. If you do the candy thing, feed them. Pass out a mini to each person and provide them with a taste test. If you don't do a food topic, try to have some kind of visual aid that can be passed around. That accomplishes a few things. First it allows the audience physical manipulation (kinesthetic) of something, and second it takes eyes off of you. That can help relieve your stress.
Under 5 minutes is not difficult. Use the motivated speaking sequence. Infomercials do it all the time. First present your audience with what their life is like without your product. Then tell them all of their problems can be solved by using your product. Show them what their life is like with your product and/or the benefits to your product. Persuade them that your product is so much better. Then tell them where they can get your product. That's 4-5 parts, depending on how you divide it up. Take 45-60 seconds per part, and there's your speech.
You listen to your teacher and have a lot of fun!