How to catch the db exception in stored procedure

Hi,
I am using Sql Server 2005. I have one doubt. I write procedure for multiple queries.
BEGIN TRAN
BEGIN TRY
query 1
query 2
query 3
COMMIT TRAN

END TRY
BEGIN CATCH
ROLLBACK TRAN

END CATCH
Now i am getting error in the query 1 or 2 or 3 . that is data base error For eg autoid parameter is not supplied . How can i catch the error? And what command is used for this one. I need this one very urgent.Hope your's reply.
Thanks