First of all u remember that a bug is nothing but what the 
end user not expected to do.

I will take an example of Yahoo mail. login to yahoo.com 
and click on the compose button. Then it should navigate to 
Compose page. if it is not going to that page then it is a 
bug. Simple!!!!

i found a bug with rediffmail signup form, in registration 
form the DOB field future month is also considerd.

You cant just describe ANY Bug that comes to your mind! You 
need to think why you remember that particular bug/Defect? 
It needs to be important enough, that defect should have 
made a big impact on the System. Maybe you could narate the 
details of a defect that led to loads of drama between the 
Test Manager, BA, Requirements Team and Development 
Manager! Or you could narate a defect that had missed all 
the Test Phases and you found it just when was about to be 
released to Production! Think of some such defect and make 
it sound real. Actually, as much as you stick to reality, 
it is that much better and there won't be any 
risk/embarrassment of getting caught.

In the DOB text box if u enter the date 29 of February and
the year is any of the year which is  not a leap year then
it should not acceptable by the application . if it is
acceptable then it is a bug .