Considering you made this thread here, in the Decomp & Disassembly Help Section, I'll assume you're talking about Pokefirered.
Scripting is no different in Pokefirered than it is in Pokeruby or Pokeemerald. They all use the same system based on macros that execute functions of code.
https://github.com/pret/pokefirered/blob/master/data/maps/PalletTown/scripts.inc
https://github.com/pret/pokeemerald/blob/master/data/maps/LittlerootTown/scripts.inc
Avara wrote a scripting tutorial too, if looking at the scripts the game contains and looking up what each command does in
asm/macros/events.inc is not good enough for you.
Are you asking how to make the character be able to run/sprint, or how to do spriting?
If it's the former, the ability to run is enabled with a flag,
FLAG_SYS_B_DASH.
If it's the latter, you just need an image editing software like Photoshop, GIMP, Paint.NET, GraphicsGale, etc and draw, while keeping in mind the limitations of the GBA when it comes to sprites.
For sprites that don't rely on a tilemap, you need to use a maximum of 16 colors, and the background color in your sprite needs to be the first color of its palette, because that's the color that the GBA will take as a transparent color.
And how to add new Pokemon and mega evolution in the game
Yeah well, you'll need to code up an implementation of Mega Evolutions for Pokefirered because no one else has done that publicly.
To insert new Pokémon species though, you can look up the tutorial in the Pokeemerald wiki for cues.
The majority of the changes presented there also apply to Pokefirered.
https://github.com/pret/pokeemerald/wiki/How-to-add-a-new-Pokémon-species
Pokefirered needs some extra changes not mentioned there though, as you'll have to incorporate any new species you decide to insert into the habitats shown by the Pokédex.
