Your professor may have given you some options, or they might have let you choose whatever you like. You can run into a few different problems at this stage:
You have no idea where to start. This is okay! Some tips here:
Browse the news
Think about your own interests. It will be easier to spend hours researching a topic that excites you. If the assignment will allow you to research and write about video games, disc golf, or origami, it is okay to start here!
Browse an encyclopedia. Encyclopedias have background information about people, events, places, etc. that might spark your interest. If you're taking a music class, for example, you can browse a music encyclopedia.
Too broad. If you think your topic has too much information to fit in a 5-7 page paper (or whatever parameters your professor has given you), you probably need to pick something more specific. Add details such as:
A specific group of people
A time frame
Your topic is too narrow. If you are having trouble finding any information about your topic on the web or through the library, it might be that you've gotten too specific. You can always check with the library to see if they have search tips, or if you're looking for information that no one's written about yet.
If you're unsure, it is usually best to check your topic with your professor early in the semester.
Last, but not least:
It is okay if you change your topic partway through. You may not realize ahead of time that your topic is too broad or too narrow. You may come across something more interesting. If you have enough time to still complete your assignment well, it's okay to start researching and then adjust what you want to do later on.