If I obtain a Certificate of Registration, shall I file return & if so, how ?

Yes, all registered employers are required to file return. Returns are annual (to be filed in the month of April for the preceding year or part-thereof) for the financial year 2014-15 onwards.
Profession Tax return to be filed in two stages
(a) Electronically: to know the procedure, you will have to consult www.wbcomtax.nic.in > Profession Tax > User Manual for e-return.
(b) Submission of signed hard copy (and acknowledgement slip) to the concerned Profession Tax authority, within stipulated time. Late submission of return attracts late fees @ Rs 200/- for the first month of default and @ Rs 100/- for each subsequent month of default or part there-of.