Check followings, the one I think the best for assembly, (You may see several other uC of PIC16 series were also used, but you can easily ported them to 877A) Nigel's PIC Tutorial Page
Thanks
I would start by downloading MPlab from the microchip web site, you get the assembler and a size restricted 'C' complier for the Microchip range. You will then have the starting tools required.