Hi again, the software I have been using for script editing is called XSE. I believe it is the most commonly used.
I cannot post links yet on this account :(
But, in the "Tools, Tutorials & Resources" subforum of this ROM Hacking forum, there is a thread called: "diegoisawesome's MEGA-HUGE XSE Scripting Tutorial"
It has been great in helping me start to understand how the scripts work, and how to easily change them or make new ones. I would say at first it can seem like a lot to take in, but after reading it again a couple of times, and taking breaks in between to experiment, you start to get a feel for the process. Really the best thing is to just keep practicing it until you get the hang of it.
I haven't figured out how to get the tools I have to add a sprite
From my understanding, it would depend how you want to go about it. If you made a new map for the event to take place on, you would first create that map and insert it into the ROM. Then on that map, create a new event, (using advancemap) using the overworld sprite of the pokémon you want to use - as the displayed sprite of the event. Then using a script editor, make this event start a battle with that pokémon when you activate it. I hope that makes sense?
Now, if the sprite you want to use is not in the game, or you want to make a new one, you would have to create it yourself and add it into the ROM. There are a lot of sprite editors I have seen, I have been using one called Nameless Sprite Editor to add in new sprites and palettes.
Essentially, you would need to look for a tutorial, or use trial and error, to learn how to insert new sprite and palette data into the game.
other threads about the question
There are a lot of threads about this concept, but many of them are very outdated now, so maybe not so useful.
Keep in mind though, I am inexperienced, and learning some of these things at the moment, this is just my best attempt to point you in the right directions.