simple example of how can you use observable to bind html text

Here is code for your html part

First name:

Last name:

this is for your ViewModel which you can use . The noticeable part here is i have used observable instead of using just plain JavaScript strings . so now when any changes made to those properties , changes will also refelect in your model . 

function AppViewModel() {
    this.firstName = ko.observable("Rahul");
    this.lastName = ko.observable("Sharma");
}
// Activates knockout.js
ko.applyBindings(new AppViewModel()); 

Here is output window screen and link of js fiddle