Where do you want the value - at the client side or on the server side code?
a) Client side - Use JavaScript to get the value like this:
var textboxvalue = document.getElementById("TextBoxID").value;
b) Server side -
String textboxvalue = TextBoxID.Text;
Here, TextBoxID is the id of the textbox whose text you want.
This is a very basic question. I suggest you to get a beginner ASP.NET book and read it before you start programming. It will help you in the long run. :thumbsup: