You should start with the basics. Here is the link to the Microchip reference manuals.
www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1956
First read this document:
https://ww1.microchip.com/downloads/en/DeviceDoc/31004a.pdf
Then, read this section
https://ww1.microchip.com/downloads/en/DeviceDoc/31005a.pdf
The main document is this:
https://ww1.microchip.com/downloads/en/DeviceDoc/33023a.pdf
This is the reference manual for the entire PIC Mid-range micros. it contains everything there is to know about these parts.
But don't bother reading it all. Just read the sections that apply to the task at hand.
The website also gives you various sections of this manual separately, so you can refer to them easily when you encounter a problem.
Finally, you can look at this applications manual (for 8-pin devices only, but you can get some ideas).
https://ww1.microchip.com/downloads/en/DeviceDoc/00711a.zip
The best way to learn is to try and build something.
If you cannot do that, at least download the MPLAB IDE from the Microchip website and try to simulate your code, or some of the examples in the applications handbook and modify them to do other things. You can watch the registers change and even the pins, like you would see on a scope. (Or you can animate your code and see how the micro executes the code, one instruction after another).
Once you get the hang of it, you will find it's really easy to work with the PICs. All the mid-range PIC's have pretty much the same instruction set (with minor differences), so once you work with one, switching to another is merely getting used to the new peripherals and memory; the ALU and instructions are the same.