How do i save my data When it comes to a win/lose leaderboard?

What you need

In order to do what you want, you need to use DataStores.

• Create your DataStore using the GetDataStore function of DataStoreService.

• You can then save keys and values to your DataStore using the SetAsync function, and retrieve them using the GetAsync function.

• You will have to retrieve saved values using a PlayerAdded event, and save the desired stat's Value using a PlayerRemoving event.

Code

--Creating the DataStore
local ds = game:GetService("DataStoreService"):GetDataStore("statz");

--Loading stats
game.Players.PlayerAdded:Connect(function(plr)
    local data = ds:GetAsync(plr.UserId);
    if data then
        plr:WaitForChild("Kills").Value = data.kills;
        plr:WaitForChild("Deaths").Value = data.deaths;
    end
end)

--Saving stats
game.Players.PlayerRemoving:Connect(function(plr)
    local data = {kills=plr.Kills.Value,deaths=plr.Deaths.Value};
    ds:SetAsync(plr.UserId,data);
end)