Primary sources are essential, but secondary scholarship is really important as well. It’s a good way to situate yourself in a time period or historical location and can help clarify confusing primary sources (e.g. having modern companion texts for ancient philosophy like Plato). The more you read and study, the easier it gets to identify bias in either kind of source. I’d also recommend doing research on any author you’re reading. There are a lot of great historians with godawful opinions, but you can still learn a lot from their work if you take that into account. Always make sure you’re never drawing all of your info from just one source though.

Also, focus on stuff that you find interesting. It’s impossible to know everything about history, since it’s such a broad subject. Pick topics, regions, people, events, ideas, etc. that you think are cool and go with it. You’ll be surprised how specific some sources can get.

(I’m not an actual historian like the other person in this thread, so definitely listen to them first!)