How do I make it so every second, your jump power increases?

Use a while loop with a wait() of 1 second, and with each iteration, increase the jump power of the Humanoid.

local Player = game.Players.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Character:WaitForChild("Humanoid")

while true do
    Humanoid.JumpPower += 5
    task.wait(1)
end

As @T3_MasterGamer mentioned, it is better to use task.wait as it is slightly faster and will not throttle either.