It's a bit long winded but here goes
A certificate of newness is a good start but you've still got a way to go. If the bikes brand new then it will be easier, however new or older you require DVLA form V55/5 and the guidance notes that support it. The form needs to be completed accurately and have all supporting documentation for submission to the DVLA. Documents include, certificate of newness, valid insurance certificate (issued against the VIN or frame number), proof that import duty has been paid and value of the machine. If the bike is three years old it will require an MOT. You also need proof of who you are i.e. birth certificate, passport or drivers licence together with the registration and licence fees.
It is likely that the DVLA will want to inspect the vehicle before a V5 and tax disc are issued (you'll need some patience here) some inspectors will check frame and engine numbers others will have a tape measure out and record seat height ground clearance for reasons unclear to me
Hope this doesn't put you off too much, prepare well and you should get on OK
Regards
John