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