Hi... and welcome to a lifetime of learning and a ton of FUN! I've been collecting over 50 years. All I may suggest is go through what you have and find something that peaks your interest and start from there.
Not knowing exactly what you have, or what you may find, in your, "...many thousands of stamps and FDCs..."; there may be singles, plate blocks, postage dues, special delivery issues, postmarks, coils, machine cancels, postal stationary, printing varieties --- or more. I guess you will have
plenty of sorting to do; by year, category, type, etc., and here is where a catalog will help.
You might find it helpful to invest a few $$$ in a "Scott U.S. Specialized Catalog". It will help you sort, understand what you have and gain insight into some of the collecting segments of the hobby.
The "INTRODUCTION" pages of SCOTT will give you a basic grounding of terms and what you should be looking at and looking for in your material. And while the "INTRODUCTION" covers the entire stamp history of the United States, a period earlier than the material you have, there is much that you will find helpful.
Also, if you have never handled stamps, etc. a few suggestions:
1. buy a good set of stamp tongs.
2. find a clean dry area to store your stamps, etc.
3.
do not store material in moist or damp areas.
4. handle material with common sense: clean, dry hands; stamps
are not cat and dog friendly (I know...I have two cats: one JUST LOVES to chew the latest version of 'label-type' stamps; the other likes to chew envelopes.

5. If you have children beware: my grand daughter spotted some loose stamps one day and before I knew it she had pasted them on the floor!
(~ I think you get the picture. ~)
Ask ten collectors "how or what to collect" and I'll you'll get at least twenty answers.
My suggestion: find
one area that interests you, and focus on that to start... THAT should likely result in hours --- years of fun. I know, it has for me.