Hi!
I think, an easy way to get the ball rolling, for the sake of inspiration at least, is to watch shows/movies that reflect a bit of the setting and mood that you're aiming for. I personally like to see how certain things have been interpreted by other creative teams, especially in terms of how they navigate accuracy vs. stylisation.
This'll likely help you paint a picture of what you might want for your story in terms of aesthetics, tone and even language/vocab. And also, it'll open up venues you can explore later in your research.
For exemple: Two pieces of media that were helpful for my project, which is a werewolf story set in colonial 18th century, were AMC's Turn, and Tim Burton's Sleepy Hollow.
Where the medieval period is concerned, here are some things that might be helpful to look into:
-Social hierarchy, what it means to be a peasant vs. nobility
-Mythology and/or religious symbols.
-Art. It says a lot about the primary values of an era.
If you're going for historical fiction, It'd be important to identify where and when you want your story to take place. It'll help focus your research. Medieval history spans just about a thousand years, and I bet you life in the 5th century was much different than life in the 14th!
As was mentioned earlier, google and books are your friend!
Hope this helps! : )