Random Player Select GUI?

First You need to make a list specifically for the players that are online

while true do
	people = {}
	for i, player in pairs(game.Players:GetPlayers()) do
		--GetPlayers() grabs all the players within the list
		if player and player.Character then
			--Checks to make sure the person is completely loaded into the game
			local humanoid = player.Character:WaitForChild("Humanoid")
			if humanoid and humanoid.Health > 0 then
				--Does the same as done before
				table.insert(people,player)
			end
		end
	end
end

You must then choose via random with math.random

Chosen = people[math.random(1, #people)]
--Do whatever with the chosen player after this line