You need some type of bill of sale to show you purchased it otherwise DMV probably won't talk to you. Once you have that it's pretty straightforward, fill out the registration form andpay the lady. There should be a HIN somewhere near the stern of the kayak. If it's a plastic/fiberglass kayak it's probably embossed into the hull itself. If it's aluminum or other material it may be riveted on there. If there is no HIN anywhere on the kayak then DMV will have you apply for a HIN which can take a bit of time.