Add an order option to magento

Hello, The following needs to be done. I have a webshop that sells pens. And i want to sell engraving. So i need function build in that people can choose if they want engraving, and if they choose yes they have to enter a text that is max 20 characters long. I have to be able to disable the engraving on some products, like inkt fillings and materials that can't be engraved. Who can build/help me with this?