You can either contact a preferred insurer directly for a quote, or use one of the comparison sites like Confused, Go-Compare ect, where it will ask you a bunch of questions (you'll likely need to play with some of the answers to get a price you can live with) and they'll show you a list and prices of insurers willing to cover you, they'll then put you in touch with the insurer one way or another. (if just getting quotes it's best to uncheck any 'please have insurers call me all day and night!' boxes on the sites.)
TBH the comparison sites tend to have pretty good FAQs and guides as to what sort of product would be best for you and how to do it.
Where is the car now and did you tax it? needs tax and insurance to be on the public road. The trouble with not getting a quote before you buy is you might get a high enough quote to make driving it uneconomical. It's a Micra tbf so you likely won't be too bad. Depending on engine there's a good chance you don't need to pay any tax, but you still need to go through the same steps and tax it, just for £0.00.
You'll also need details of your driving record (when you got your full license, NCB, and any accidents or driving convictions) and the car (including exact model and modifications, how it will be used and where it will be kept).