Start with a general book on music history that with all the major time periods --- from Ancient Greece to contemporary music. Listen to the musical examples of each time period.
Learn about specific composers and how their music influenced others. Listen to their music and delve into the techniques that made the music important. Become familiar with the names of the most important composers for each time period.
Enroll in a course at a local community college or university.
Read a textbook that chronicles the history of music from the beginning. Recognize that often in music history, earlier topics underlay the significance of later events. Composers often find influence by innovations from a previous period of music.
Return to areas that are unclear. Look for additional resources and references to get a deeper knowledge of the subject.