First determine who your audience is. If it is for History class, you would need to pick something historical. If it is for English, pick something about literature or poetry. If it is open, choose something you like, but make sure you have a point to make.
Pick something that you’re comfortable talking about. Although speeches are formal and structured, your speech will come across better if you are relaxed. So, it helps to talk about something you know: from past experience and/or from research you're going to do for this speech. You want to convey a sense of comfort as if you were talking to your friends. I give the same advice to students when they write papers. Don’t get bogged down with adhering to the form and structure you think your teacher is looking for. Say it in your own words and then tighten it up.
If you are giving a speech, I assume you will have a point to make. For example, if it is an informative speech on global warming, you need to communicate why you’re giving this information and what the audience can do with it. Let’s say you are giving a speech on what we can do to stop global warming. Research the causes and the solutions. Then set up your argument with some kind of attention grabber like an incredible statistic or some anecdote. Go from that into an introduction. Then present the evidence for and against your argument. Then form a conclusion. Remember that if you want this to sound legit, imagine you are really trying to persuade your friends with this speech. Like I said before, make sure the speech is polished, but write it and speak it in your own words. Talk to them; not at them.
for any writing or speaking project i always start with a brainstorm list. think about who the audience is, how to hook the audience in (intro), main points you want to cover, and the facts that correspond with them. Once you have all this information in one spot its a lot easier to build up your speech and cover everything you need to include.