Take a minute and think about what interests you the most in the class you need to write a paper for--or, if you're not in a class, then think about what area interests you the most. Remember that if you aren't interested in your subject, your readers won't be either. If you lose the interest of your readers, then it won't matter how much research you did, since no one will want to read your paper. (If you're in a class, then keep your teacher in mind as you write. She is your audience.)
Once you have some idea of what you're interested in, do a little research on the topic. Try to find answers to questions you have. If you have those questions, your readers will, too. Don't talk down to readers, and don't give them information they already know; writing teachers often cross this sort of thing out as fluff, and focusing on the general doesn't help you come up with a good idea. The first bit of research you do is the most important, because it helps you see whether you really want to write a paper on this topic. You might think something is interesting, but then after reading a lot about it, you might think otherwise. If that happens, go back and try to think of something else. But don't be afraid to start over. Too often, writers feel invested in an idea even when they no longer think it's any good.
But coming up with good ideas is hard work. If you're having trouble thinking of a topic for your paper, pick up a magazine or newspaper article in the area you need to write on. Don't read the whole thing necessarily, but flip through and try to find something that interests you. Not all writing problems come from lack of interest though. Be persistent in your search for a good idea. Read your textbook or extra articles your teacher gave you or search online sources such as The New York Times archives for your topic. Be diligent about setting aside time to come up with your idea. Diligence and persistence will also pay off once you start developing your idea in writing.