My first real-time effect must have been some kind of effect for a forgotten Quake 1 multiplayer mod. So forgotten I can’t even remember the name of it myself. Basically it was a mod that replaced all of the weapons with sort of fantasy themed weapons (most of which still shot something), and came with a CTF like game mode where you were supposed to kill the other team’s snub-niggurath. I remember making a custom effect for one of the weapons, a kind of lightning hammer skull thing. It was built with an archaic hacked together Quake 1 effects system which I don’t remember at all anymore.
My first professional effect could be considered the faux atmosphere effect for the NASA Virtual International Space Station from 2001.
I did that effect by having a simple faded ring facing the camera, and having it float in front of the 3d sphere planet. I still use that trick to fake volumetrics, so I guess it’s still relevant?
Around the same time I did a water fountain effect for a German bank arch viz using Unreal 1’s animated texture stuff. Completely irrelevant today.
However I did those as an “unpaid” contractor and was paid in bits of PC hardware, so it’s debatable if that should be considered “professional” work. I did several bits of fire and smoke for the Harry Potter and the Sorcerer’s Stone PC game that same year, so that’s probably more likely my first real professional effects work.