function startMonitor()
   monitor = peripheral.wrap("top")
   wm, hm = monitor.getSize()
end
startMonitor()

local cwrite = function(text)
                local x2,y2 = monitor.getCursorPos()
                local x,y = monitor.getSize()
                monitor.setCursorPos(math.ceil((x / 2) - (text:len() / 2)), y2)

            monitor.write(text)
end

cwrite("Hello World!")  -- centers printed text to monitor