@lnl555 

Welcome to the wonderful adventure that is Backpacking!  I am sure your local REI would have some in person (or currently "Zoom" type classes), but you can do a quick google search on what to be prepared for.  See below for a couple of links:

https://www.rei.com/learn/series/intro-to-backpacking

https://www.outsideonline.com/2394143/beginners-guide-backpacking

As far as hikes in the DC area, I have been to McAfee Knob....it's a great little area with some breathtaking views.  Several overnighters in that area.

A few questions I have for you though...do you already have your Big 3 (Tent, Sleeping Bag, Backpack)?  Do you have a cook system?  What time of year are you looking to go (your Big 3 could turn into a Big 4 or 5 depending on the season)?