You will have to pay penalty as Tax amount evaded + INterest +Penalty.
1)In your dashboard, you will get an option "reply to notice" there you will get an option to attach also.
3)a) In this you have to show the proofs of investments you made like LIC, PPF, Mutual Fund etc
b) If you have house property loan interest then only you will get deduction. You have to submit photocopy of Amount paid
c) Salary certificate Form 16 you have received from employer.