How to apply validations or cutomize OOB New forms, Edit forms, Disp Forms in Sharpoint 2010?

SharePoint List Form Validation with Jquery:

While we have our list opened in our browser we can use the List Tools to open the Default New Form. This is the form where we want to add our code

 

  

We are presented with a design view of the new form. We need to add a Content Editor Web Part so we have a space to enter our JavaScript code

 

Add Content Editor Web part

 

 

Add your JavaScript or Jquery Code inside Content Editor Web part

Now open the page  it will show the alert box