Use irons for the first while. You should already have a decent set on your gun, or you can buy decent Airsoft flip up back ups for fairly cheap.
If you feel you want to upgrade to the world of red dots and glass optics, buy real steel. Buy that cheap 80 dollar bushnell or whatever. Never, ever buy an Airsoft optic. Ever. Just don't do it. I've had nothing but problems with them for YEARS.