How to add EXP to a player over time?

Here, we're going to use what's called a While Loop. Basically, what it does is it loops over a block of codes over and over again.

Check this out to know more about loops: https://developer.roblox.com/en-us/articles/Loops

BTW, back to the issue, here's how I would do it...

local players = game:GetService("Players") --gets the players

players.PlayerAdded:Connect(function(player) --checks if a player joins
	local stats = Instance.new("IntValue", player) --creates the leaderboard
	stats.Name = "leaderstats"

	local exp = Instance.new("IntValue", stats) --creates the exp
	exp.Name = "exp"

	while true do --the while I was talking about
		player.leaderstats.exp.Value = player.leaderstats.exp.Value + 1 --increases exp by 1 overtime
		wait(30) --waits 30 seconds before increasing exp by 1 again
	end
end)