How to create a pop up to get user input

Hello experts,

I have a requirement, in the component BT120H_CPL in the view BT120H_CPL/ItemList i have created a custom button called 'Split',

when the user selects a line item and clicks on this button; i need a pop up which asks the user "The number of splits required" and there should be place for user to enter the answer, this answer will be used for further process. How can i achieve this pop up? i have never worked on pop ups. please help me

Thanks,

Deepak