Welcome.
If your ecu is pro flashed then chances are it has some form of protection on the tune which could cause it to be bricked if you play with it. If you Google "bootmode Nefmoto" you'll see lot's of ways people have got around any bricked ecu's and any of the other ones you play with, generally building them off a stock base file.
Following the Stage 1/2 threads in the Tuning section is your best bet to get to know the basics and, of course, the infamous wiki explains the various aspects succinctly;
https://s4wiki.com/wiki/TuningI think the main thing you may struggle with are locating the codewords for switching things off, but there are quite a lot of def files knocking about on the forum which should show you or make things easier - a good definition file is what you want!
Another quick and cheap(ish) upgrade is to slap on the k04 stuff, turbo set up, maf and injectors, then you can flash on a stock 225 map and code out the bits you aren't using or are different on your engine, alternatively just follow the threads and tune whatever set up you put on.
One last thing - you need to be able to checksum your file after changes, this can in-built in editors like winols, they can be found in the more modern flashing tools like MPPS v18/21, a standalone cs one like ecuedit or manual changes, although automatic is far quicker if you're making multiple changes!
