How can i make a seat event that fires when someone seats on a block?

You can use GetPropertyChangedSignal:

local Seat = script.Parent

-- GetPropertyChangedSignal fires when the given property (in this case "Occupant") changes
Seat:GetPropertyChangedSignal("Occupant"):Connect(function()
	local Occupant = Seat.Occupant
	-- Seats have a property called occupant. When a npc/player is seated, the occupant is set to their humanoid
	local Player = (
		Occupant
		and game.Players:GetPlayerFromCharacter(Occupant.Parent)
	)
	if Player then 
		print(Player)
	end
end)