Create an application in salesforce.com using APEX programming language

Step 1:

Log into your Sandbox or Developers Organization.
Click on setup ⇒ create ⇒ objects ⇒ new custom objects.
Enter Book for label.
Enter Books for plural label.
Click Save.

Step 2:

Now let's create a custom field.
In the custom field & relationship section of the Book Object click new.
Select Number for the datatype & next.
Enter Price for the field Label.
Enter 16 in the length text box.
Enter 2 in the decimal places & Next….next…. save.

Step 3:

Clilck setup ⇒ Develop ⇒ Apex Classes & click new
In the class Editor enter this class
   public class MyHelloWorld{
   public static void applyDiscount(Book__c[] books)
   {
   for(Book__c b:books)
   {
     b.Price__c*=0.9;
   }
   }
   }

Step 4:

Add a trigger
A trigger is a piece of code that can execute objects before or after specific data manipulation language events occurred.
Click on setup ⇒ create ⇒ objects ⇒ click the object you have created ex: Book
Scroll down you can see Trigger Click on New
In the trigger Editor enter this class
trigger HelloWorldTrigger on Book__c(before insert)
{
Book__c[] books=Trigger.new;
MyHelloWorld.applyDiscount(books);
}

Step 5:

Click on setup ⇒ create ⇒ tabs ⇒ new custom tab ⇒ choose Book ⇒ next….next…..save.
Click on tab Books ⇒ new ⇒ insert a name for Book ⇒ insert price for that book ⇒ click on save.