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:
Look carefully at your assignment instructions before brainstorming. What is the end goal of your paper/project?
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.
You have an idea, but it is...
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 particular group of people
A time frame
Too narrow. You may not realize this problem before you begin searching and find very little published on your topic. Check with the library for search tips or help with refining your idea.
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.