can anyone help me on this script?

function onPlayerEntered(newPlayer)
local stats = Instance.new("IntValue")
stats.Name = "leaderstats"

local Mana = Instance.new("IntValue")
Mana.Name = "Mana"
Mana.Value = 100
if Mana.Value>500 then
Mana.Value = 500
end
if Mana.Value<500 then
Mana.Value = Mana.Value + 10 --Adding 10 to "Mana" everytime this statement runs.
end

end
game.Players.ChildAdded:connect(onPlayerEntered)

This is doing what you asked for, however, I think your overall goal may need a lot more work in the future.

I wish you luck in finishing your idea!

EDIT: BlueTaslem has a better answer as well as a more efficient way of accomplishing what you intend to do.

You want to increase by mana by ten there? (This is called incrementing by 10)

We set Mana.Value to what Mana.Value was + 10:

Mana.Value = Mana.Value + 10;

Note that the "increase by ten but cap at 500" can be written as a single line without any ifs:

Mana.Value = math.min( 500, Mana.Value + 10 );
-- This replaces line 08 to 13 in the snippet you posted

That is, the lesser of 500 or increasing Mana.Value by 10.

Note also, however, that that work has to be done continuously. Right now are you just doing it when the player joins, which will not affects things later. It should be in a while loop with some wait.

Something like:

while wait(3) do
    -- Every three seconds . . .
    Mana.Value = math.min( 500, Mana.Value + 10 );
    -- Recharge by ten
end