History is an extremely diffuse subject, and it is, I'd say, pretty much unique among the HASS [Humanities, Arts and Social Sciences] disciplines in having no real "canon", a core of books and a set of ideas or approaches which everyone involved in the field is expected to have read, and which collectively provide a set of touchstones and a group of theories which are familiar to all. In fact, while there certainly is plenty of "theory of history" material around, quite a high proportion of even professional historians don't involve themselves overly in theoretical, as opposed to purely methodological, debate.

There is always more to say, but AH does offer some starter resources that might help begin to answer your questions, and you might like to review some of those while you wait for fresh responses:

How do historians start out?

The AskHistorians FAQ on history as a vocation (also includes numerous resources on finding and working with sources)

Dealing with accuracy, exaggeration and "bias"

Historians coping with bias, with u/DanKensington

The starting point for historical research

I am trying to do some historical research, and it is overwhelming me. How can I handle things better? with me, u/mikedash

How to read an academic book, with u/sunagainstgold