There are many ways to do this. The first thing you should do is learn enough LabVIEW to be familiar with the Palettes (such as the String Palette), have some idea of how to check for "only file name characters" (I can think of several ways, depending on whether you want to check as each character is entered, or only when they all are in). I don't know what you mean by "Enable" a button, but several methods are available, including "hiding" it by making it "invisible" (you'll need to learn about Control Properties and how to manipulate them). When interacting with the User Interface, Event Loops are often helpful.
Take some Tutorials (check the list on the first page of this Forum), take a class, find a Tutor/Mentor.