DATA STORE API?

Data Persistence is not a reliable way of saving leaderstats, your question doesn't seem understandable. I will still explain Data Store though.

Data Store is a great way to save stats of players and is very easy to understand. I will first explain on how to get a player's stats because this is very important, and calling the service.

We always must define the service that we will be using.

local Datastore = game:GetService("DataStoreService")

Next we must add a on join function

local Datastore = game:GetService("DataStoreService")

game.Players.PlayerAdded:connect(function(player)
    --The code will go here
end)

Now we must create the player's stats, please note that you can change the stats to anything you want.

local Datastore = game:GetService("DataStoreService")
local Values = {"Cash", "Exp"}

game.Players.PlayerAdded:connect(function(player)
--
local data = Instance.new("IntValue", player) --Creating leaderstats
data.Name = "leaderstats"
--
for _,v in pairs(Values) do --Using In Pairs
        local newvalue = Instance.new("IntValue", data) --Creating new IntValues
        newvalue.Name = v --Naming the new IntValue
    end 
--
end)

Next we should wait for the player to load.

player:WaitForChild("data")

Now we must load the player's data if they have some, and save the stats when they change.

local Datastore = game:GetService("DataStoreService")
local Values = {"Cash", "Exp"} --Stats

game.Players.PlayerAdded:connect(function(player)
--
local data = Instance.new("IntValue", player) --Creating leaderstats
data.Name = "leaderstats"
--
for _,v in pairs(Values) do --Using In Pairs
        local newvalue = Instance.new("IntValue", data) --Creating new IntValues
        newvalue.Name = v --Naming the new IntValue
    end 
--
player:WaitForChild("data")
--
local Data = Datastore:GetDataStore(player.Name.."Stats")
    for _,v in pairs(player.leaderstats:GetChildren()) do
        v.Value = Data:GetAsync(v.Name) --Getting the value that has been saved
        v.Changed:connect(function()  --When one of the values change
            for _,i in pairs(player:FindFirstChild("leaderstats"):GetChildren()) do
            Data:SetAsync(i.Name, i.Value) --Save the stats
            end
        end)
    end
--
end)

I hope that you can understand from this example, If you still don't understand then you can look at the link below.

http://wiki.roblox.com/index.php?title=Data_store

If this is considered your answer, then mark it as answered!

This is as good a place as any to start, since it seems you've not yet done much research on the subject.

@RoboFrog

Don't forget DATA STORES