A few good books are:
1) Understanding Exposure by Bryan Peterson
2) As an alternative to #1, some people prefer The Digital Photography Book, Part 1 (it's a series and I think it's now up to 5 books) by Scott Kelby.
3) The Photographer's Eye: Composition & Design for Better Digital Photography by Michael Freeman
4) Speedliter's Handbook by Syl Arena (this book is very good, but is specific to understanding the Canon ETTL system -- though it does have many good tips on effective use of flash photography... it is nuanced toward Canon's system.)
5) Light Science & Magic: An Introduction to Photographic Lighting by Fil Hunter, Steven Bifer, and Paul Fuqua
#1 and #2 are very commonly recommended to new photographers as they teach the basics in plain language that doesn't require special expertise or previous experience to understand. It isn't just collecting the right amount of light for the exposure... but how your camera collects it that will influence the result (will you have a tack-sharp subject with a blurred background? Will you have relatively sharp focus of everything in the image? Will you freeze action to a moment in time? Will you deliberately blur action to imply motion in the shot? Etc. and how to know which approach to take to get the creative results you want.
The Scott Kelby book (#2 on the list) is an alternative... some people prefer the way one author writes vs. the other. But the Scott Kelby book is the first in a long series.
I always recommend #3 ... once you understand the basics (in #1 or #2). This book doesn't tell you how to adjust camera settings... it teaches you how to recognize elements in the scene that make for good photography.
#4 and #5 are both about use of flash. Good lighting has a huge influence on your images. These books teach you how to control the flash for much better light, get the flash off the camera so that you can not only control the highlights, but also the use of shadows. Lighting probably wields more influence over the look of an image than the choice of lenses.
So back to post processing software...
Lightroom is the most popular software used (by far). But what's probably stumping you is that you aren't sure what to do when you open Lightroom. You probably want to watch some tutorial videos.
Canon's Digital Photo Professional (DPP) comes with your camera (it's free to you) and it's designed to let you adjust your RAW images (you could adjust any image, but it's optimized to deal with Canon's RAW files). But you do edit these files one at a time.
Lightroom is not just a RAW processing program... it's also a Digital Asset Manager (DAM) program. If you do a lot of shooting and you aren't organized, you may have photos all over your hard drive and you might struggle to find them. Lightroom starts with it's "Library" module which controls how it imports and stores the original files and allows you to sort them into "collections" (think of collections like a photo album for a theme or event, etc.) For example, I professional wedding photographer probably has a "collection" for each wedding or each client. I have some common locations that I like to go to shoot so I have "collections" for those locations (each time I go to that same location I typically add the images to the same collection). It also allows you to do keywording of your images to make it possible to search them quickly. It lets you rank your images. So you can do searches such as finding all the shots taken with a particular camera & lens that you ranked 5 stars and are portrait shots. When you have tens of thousands of images... being able to do fast searches is really handy.
It's also a RAW workflow and image adjustment program.
When you shoot an image using JPEG, the camera does a lot of processing on that image (in the camera) before it saves the image to the memory card. A JPEG typically gets "white balance" applied (to deal with the color cast of the light); some de-noising probably occurs if you shot at high ISO; some sharpening probably occurs around edges; a color style is applied, etc.
When you shoot using RAW, none of this happens (even though most images need some adjustment). But JPEG is a "lossy" storage type which means subtle differences from pixel to pixel will be dropped to make the image compress better for storage. If you then later try to adjust the image, you usually find that some detail was lost in the compression (and you can't recover it -- hence the name "lossy"). RAW images don't discard or adjust data - you get everything the sensor saw.
Programs like DPP and Lightroom let you apply all those things that JPEG would have applied automatically... but since RAW isn't "lossy", it is VASTLY better for post processing / adjustment. You can recover far more detail in your images as you work on them.
So your question about "where to start" is answered by two things... (1) is probably some tutorials in how to use lightroom ... but the other thing to learn is (2) what are all the common adjustment types and why would you use them?
The answer to #2 is that once you take a photo, it will look a LOT better once you process it (take it through the post-processing workflow).
Here's an example:
This is a before/after comparison... the 'before' is basically how it came out of the camera. You can see it's a bit washed out, there's some clutter on the left side of the frame... I cropped it, I adjusted the "white balance" (it looks a bit cold with a slight 'blue' color cast based on the clouds), I checked and adjusted the 'white' and 'black' points in the image to improve contrast. I adjusted the tone-curve for the image which further 'stretches' the contrast (instead of this image having an fairly flat gray-ish look... you can see the output is a bit more dramatic with both brighter and darker areas instead of mostly just middle-gray areas). Some sharpening was applied (which you probably can't see in this tiny sizes but do show up in a full size image) and there's even a bit of vignette adjustment do darken the outer areas of the frame and bring attention toward the center.
Anyway... this is the general idea of what is meant by "post processing" ... you're taking your original data and tweaking it to get a result more to your liking. If you try to do this with a JPEG image, you'll find lots of issues because JPEG scrapped lots of original data. This is why most serious photographers shoot RAW.
Programs like Photoshop can do this (but Lightroom is faster at THIS type of editing) but Photoshop will let you go beyond reality... you can insert elements into a photo that weren't really there (e.g. add some people, remove something from the background, etc.) Photoshop will let you apply effects (create blur where something wasn't really blurred, etc.) Photoshop lets you do the sorts of things a graphic artist might do (beyond photography.)
I do use Photoshop... but only very rarely ... probably more than 98% of what I do with photos can be handled with Lightroom.
One of the other cool things about Lightroom is that if you take a lot of images in the same lighting with the same camera, etc. then there are probably a lot of adjustments that should be made to the entire set of images. In Lightroom you can "sync" your adjustments... adjust one image ... then select a range of images and tell it to "sync". Lightroom will open a panel showing check-boxes for each adjustment type (e.g. "white balance" would be one checkbox). You can decide which kinds of adjustments you want to sync and when you perform this, those edits will be applied to EVERY image in the range you selected... this really speeds things along when you are working with a lot of images.
