If you want to write C you'll need to install the XC8 toolchain - mpasm cant compile C. You can download it free from the Microchip website, google it and you should find it easy enough.
You'll also want to create a "main.c" file instead of a text file - you will need a file called main.c with a routine called main() within it. That is essentially where the microcontroller will start executing, so you can call your reset routine from there to initialise peripherals etc.
The easiest way to do that is (after installing XC8 and restarting MPLAB X) is to right click Source Files, hover over New and then click main.c. That will create the file with all the requisite minimums.
I used to write assembly for 8 bit PICs back in my early days - I do enjoy it, its a fun challenge, but these days I have also moved to C pretty much exclusively. Its a lot easier to read (if written well) and way more productive.
My projects tend to follow this kind of structure:
Header Files
- config_bits.h
- main.h
- file1.h
- file2.h
Source Files
- main.c
- file1.c
- file2.c
config_bits.h contains a copy/paste dump of the output of the config bits tool. It is included in to main.c
main.c/.h contain the main() routine which usually contains an infinite loop with a state machine if the project is complex enough, and maybe some other generic helper functions and (in the .h file) global variables. I try not to put too much in to main.c, instead breaking things out and grouping them in to other source/header files as appropriate.
file1/file2 .c/.h would be examples of where I would group related routines - for example, routines may be grouped by the type of peripheral they relate to, like the UART, I2C, etc. I'll typically use a much more descriptive name than just file1, file2 etc.
There are probably a whole bunch of schools of thought about how to organise your project - this works for me, use what ever works for you, or follow established guidelines (or establish them!) if working with a team. Look at other projects and see how they do it, like the Linux kernel.
Oh and pick
1 coding style and stick to it!