Hi there, that’s a big question 😊!
You can use a domain registration site like https://nz.godaddy.com/offers/domains to research website names and register the web address. Then WordPress (lots of cheap website templates available https://wordpress.com/themes, https://elementor.com/pages/free-templates-3, https://themeforest.net/category/wordpress). Or you can use a website builder tool like https://www.wix.com/.
I recommend doing some keyword research first (https://moz.com/explorer is helpful and has a free 30-day trial, other tools can be found for example https://www.semrush.com/lp/keyword-magic-tool-1/, https://ahrefs.com/keywords-explorer, but I find Moz easiest). Use these tools to understand what sort of terms are used in your target industry and try and use a related, memorable and easy to spell/type term for the website domain name.
In terms of the legal and process side of things, Here is a link to several start-up templates that are good starting points https://kindrik.co.nz/resources/startup-resources/.
Regarding indemnity and insurance, any business insurance broker can help you out. https://apexinsurance.co.nz/ is an example that I have come across in my past, but any business insurance broker will help you out. (Quick comment, if you haven’t started the business yet, insurance can probably come a bit later but please take your own advice on this).
Regarding marketing, it depends on where your target market is. Can you provide a little bit more insight?
I have posted this elsewhere, but this is a useful beginners guide to getting found online (at least in search engines). https://www.skillshare.com/classes/Introduction-to-SEO-Tactics-and-Strategy-for-Entrepreneurs/681600915
Understanding what your customers read, where they hang out, where look for best practise and answers to problems, asked for vendor recommendations, etc. will hopefully you understand where to start your marketing efforts.
All the best. Reach out if you need anything else.