You have to Add WebService in App_Code folder like this
Then create one Insert WebMethod function in WebService. Then In Aspx page on Button Click event Create the Object of Service class like this:
WebService obj = new WebService();
string name = this.txtUserName.Text.Trim();
int balance = int.Parse(this.txtBalance.Text.Trim());
string city = this.txtCity.Text.Trim();
obj.Insert(name, balance, city);
WebService is the class name and Insert is the function name to which i am passing the parameters that have to be saved in Database. You have to write your Webservice class name properly otherwise it will give you error.
Thank You.