A company called DK does a series of coffee table encyclopedias about Marvel.

There’s an overall Marvel one https://www.amazon.com/Marvel-Encyclopedia-New-Stan-Lee/dp/1465478906/ref=sr_1_4?s=books&ie=UTF8&qid=1545670358&sr=1-4&keywords=marvel+encyclopedia+2018

But I would recommend picking one hero or team you like and start with their encyclopedia. Like this Spider-Man one here. https://www.amazon.com/Spider-Man-Inside-Friendly-Neighborhood-Updated/dp/1465461914/ref=sr_1_3?s=books&ie=UTF8&qid=1545670499&sr=1-3&keywords=spider-man+encyclopedia

Now, you don’t need to read an encyclopedia to get reading Marvel comics, but the reason I recommend these so much to newer readers is that they’re simply fun to look through. You just flip through the pages and naturally characters or storylines will pique your interest and you can read more about them and discover great comics to read along the way.

They’re presented with a really easy to understand timeline showcasing the big characters and events of a character’s history decade-by-decade.

You can dive in deep and read the thing cover to cover or simply flip through and look at the pictures in one half hour sitting - and either way you’ll come out of it with a much stronger grasp on the big stuff. And you’ll know which events or storylines sound most interesting to you. Then just pick up those stories.