Form drop-down list option automatic change

I have a pop-up form like thing where when I enter the phone no. of a customer in the "Phone" field, the name is automatically input in the "Name" field. There is also a drop-down list which lets us to select the type of the customer. At the moment, there is a default selected. I also want the type to be changed to the correct type automatically when I enter the phone no.

Please tell me how to do this. Which web programming language to use and example of codes, and steps. I am just a beginner in web. Please accept my apologies if this is a rubbish question. Thank you very much.