Why not online for first timer?
You may not file your tax return in digital form if you lived in the Netherlands for only part of the year because you immigrated to or emigrated from the Netherlands in 2015. In that case, you must use Form M.

How to get M form?
If you did not receive an M Form from the Tax and Customs Administration and you wish to file a tax return for 2015, you should request a Form M.
Phone Number – Call this number(055 538 53 85). Press 1 and the someone will pickup your phone and you may request for M form. they will asl your BSN number. 
URL –http://www.belastingdienst.nl/wps/wcm/connect/bldcontenten/belastingdienst/individuals/tax_return/2015_tax_return/requesting_a_postponement_for_filing_a_tax_return/requesting_a_postponement_for_filing_a_tax_return

How to request a postponement?
You may request a postponement in writing. We must have received your letter before the tax return deadline. This date can be found in your invitation to file a tax return. If you must file a tax return before 1 July, you may be granted a postponement until 1 September.

Know more about M Form

Info about Tax Return 2015