For topics that are new to you, start with definitions. Also consider using encyclopedias at this stage.
At this level, you're ready to move on to background information. Books excel at this as do many websites. Even people can provide background info. Remember to check your source though! Books, websites or people, any one of these can be excellent or horrible.
To find out what the current thinking is on treatments and diagnosing, look to journal articles. (Often called "the medical literature", or "the literature".)