To earn the All's Fair trophy in Red Dead Online, you simply need to thwart another player or team's attempt to complete a Free Roam mission -- i.e. a non-story mission that takes place in the shared world with all players.
The easiest mission to counter is a stagecoach robbery, which will appear as a red reticle on a black circle background on the mini-map. Head over and deal with whoever is in the area, which may still be the original NPCs or may be a group of players (hopefully you catch them off guard in these early days, and it's just the one guy).
In the early game you may not have anything to destroy the coach with efficiently such as dynamite. Your best bet is to kill or free the horses, drive the whole thing off a cliff or, my method, drive it into the deep part of a river, lake or the sea. The horses will be freed and the stagecoach technically destroyed.