If you're talking about being an expert on all history everywhere, you first, you need to have a history encyclopedia at the ready or know where one is located in your local library or college library.
Online, bookmark sources for a comprehensive timeline (not all of them will have everything on it, so you'll want to use several).
Don't read just one biographer of any particular historic figure, or one single report about an event in history. Regarding wars, read the point of view of the victor, the point of view of the vanquished, and the point of view of an uninvolved neighboring nation and then triangulate a picture of what happened. Never rely entirely on a single source.
Remember that a historic figure didn't live in a vacuum. He interacted with other people and made decisions based on his environment of people, places and events. so examine the nature of the society he or she was part of. Examine the perspectives of rivals, partners and mentors; examine fellow soldiers he served with, and the like. Examine the sensibilities and proprieties considered to be the norm of the day.
In examining the details of an historic event, use the encyclopedia to examine the situations previous to the occurrence of the event--the run-up to the event--to fully understand the issues involved by the time the event happens.
You'll also need to use this reference to get a broader view of situations because it's very easy to get lost in minutia of detail. Don't underestimate the importance of minutia, though. Skipping over it entirely can make a huge difference in how you understand what happened. What's important is to just avoid getting lost and tangled in the weeds while maintaining your sense of proportion, perspective, and direction as you navigate the past.
Familiarize yourself with Project Gutenberg online. Through Project Gutenberg you get access to original writings (primary sources) of historic figures as well as access to libraries across the globe. Just be aware that primary sources present only one perspective and are written with a particular target audience in mind (for example, who is a letter addressed to? Is it a speech that is for the purpose of persuading the listener, a singular particular listener? Who? Is it a sales pitch? etc. No primary source is more authoritative than the writer's own goal for writing what was written. Definitely get a second and third opinion).
Read a lot of past issues of National Geographic at your library on whatever topic you're pursuing (or not--sometimes just exploring back issues will initiate an adventure when you weren't even looking for one). There will be archaeological updated information covered, which brings up another concern about new information compared to old information. Things that are old discoveries can be destroyed by subsequent war and decomposition, so it's a mistake to think that old books don't have value. You can discover things in those that get forgotten, buried, lost, blown up as time goes by. No archaeologist can reconstruct history by digging where a bomb dropped.
Lastly, but just as importantly, realize that language changes over time. If you're reading old books to get the feel of a past era, you must be aware that even though the words may look familiar to you, the meanings are different, so be sure to have handy a thesaurus published with a copyright date near the time period you're researching.
Best of all--have fun! There are exciting adventures to be had!.
Experienced time traveler.