It depends on what you want to do. Do you want to create audio or just add in audio already made by other people?
If you're talking about creating audio...
Custom Audio:
Like you said, it's as simple as finding the audio you want, ripping it, and adding it into a folder. So for example, lets say you rip an audio file and you want to use it as Goku's Kamehameha sound, you'd simply have to go to the ESF folder that contains Goku's Kamehameha sound, rename your file so it has the same name as the Kamehameha file, and then replace the file in the folder with your sound file. Next time you start up the game, your sound will be played.
Sprites
If you're interesting in creating sprites, know that it takes skill and practice.
Read over this thread if you want to learn more about creating sprites. After your sprite is created, it's just a matter of replacing a sprite in the ESF sprite folder with the sprite you've made.
Mapping
Mapping takes a lot of skill and practice.
Check out this thread for some information, or use google to get some more info.
Animating
Again, this takes skill. I couldn't find a tutorial thread for this on the forums, but use google if you're interested, I'm sure there are a lot of tutorials out there.
Models / Skins
Again, this takes a lot of skill and practice.
Check out this thread for some information, or use google to get some more info.
Coding
In order to make changes on the coding front, you'd have to do it through amxx. That means
you're going to need to learn how to code in Pawn. Sounds challenging, but Pawn is actually a relatively easy language comparatively speaking.
If you're talking about adding in stuff already made by other people, you'd have to wait until
ESF-World is back up which should be in another month or two. And that's just a matter of downloading a file and replacing the file in the ESF folder with the file you downloaded.