I started with an Action Replay (which is actually what got me into Pokesav.org, which is what this site used to be). Later, I got a trainer toolkit that let me create codes. Unfortunately, it broke before I could make anything big with it. However, it was a starting point, getting me familiar with hex and hex editing.
Then I got Pokémon Mystery Dungeon: Explorers of Time, and since no one made a save editor for it, I took an interest in it. Years later after my programming skills developed more, I tried to make a pkm editor, when I got an epiphany about the checksum for Mystery Dungeon (it's similar to what's used by pkm files, except doubling the size of the checksum, and the words used to calculate it).