-- Guess I'll comment my script some too... Start up of the program
print("Welcome to mah calc!")
print("This calculator cna only do + and - ...")

function HandleCommand() -- functions must be declared before they are called, so moving functions up
    print("Please input a number!")
    local f = io.read()
    print("Please insert command.")
    local x = io.read()
    print("Please insert second number")
    local g = io.read()

    DoMath(f, x, g)
end

function DoMath(one, two, three)
    -- This will be called on startup. You want it to be called when you have handled a command. Moved to a function.
    if two == "+" then
        print("Your Input:" .. tostring( tonumber(one) + tonumber(three) )) -- you need to convert to numbers
    elseif two == "-" then
        print("Your answer is:" .. tostring( tonumber(one) - tonumber(three) ))
    elseif one == "-1" then
        print("Secret code found. Deploying nukes.")
        running = false
    end
end

local running = true
while running do
    HandleCommand()
end