When choosing a topic, start by browsing through reference books and other introductory resources. This helps you get a quick sense of the subject and to identify important related topics.
The best way to start is with a simple search on your subject's name - for example, Marie Curie.
Here are three online reference collections, each of which contains hundreds or thousands of dictionaries, encyclopedias, and biographical resources.
You can also include other types of introductory resources in your search process.
WIkipedia is a very good source for finding background information on an individual or topics, especially if it is someone whose work is more recent.
Also, for those who are still active in their fields, you may find introductory information on your subject's professional web page, or that of their institution or research group. These can often be found by a web search on your subject's name.