Some comics have decades of storylines
Use databases to find important storylines
Let's say you are interested in the X-Men; you'll find there are multiple titles and decades of storylines.
The mutant world is a confusing one indeed, especially when alternate universe titles also occasionally coincide with the main universe.
However, using a reference source such as The Comic Book Database, you can more easily discover all titles related to the XMen, their chronology, and summaries of some storylines so you can not only begin to understand the backstory, but figure out which titles you should be reading and which you can skip.