registration

Form for registration is Form ST 1 & not Form ST 3. You need to submit Form ST 1 duly filled along with self attested copy of PAN card and an address proof.

ST 1,PANCARD,ADDRESS PROOF

along with the above mentioned details, they may ask for authority letter also, only if you are going instead of your client, its a normal letter saying this person is given authority in my place

ST-3 is a form for Service Tax return. ST-1 is an application for Service Tax registration along with Pancard & Address Proof.