My first (anti-)tip: Don’t do it because of Corona. Don’t start writing because you’re bored. Nothing significant will come of it.
If you want to publish traditionally:
• You don’t need to write the whole thing to send it out to agencies or small publishers. Write a few chapters first, polish them and see if there’s any interest. If you get responses, you’ll still have enough time to finish your project.
• Make sure your extract and cover letter are proofread and have gone out to a few beta readers or even a freelance editor. You will develop blind spots when it comes to your book.
• Look for agencies that have published similar books like yours. And mention these in your cover letter!
• A publisher is not a guarantee for good sales. Expect to do a lot of self-marketing and spend money for advertising measures.
If you want to self-publish:
• Please hire a freelance editor and a graphic designer for your cover. No one will buy a book that features a terrible Word font on the cover.
• Spend 25% of your time on writing, 75% of your time on marketing. You have to take on the role of a social media manager/marketeer, sometimes even a salesperson at a publishing house.
• Expect to spend quite a bit of money for advertising measures. Maybe put aside some money for this purpose. Expect to spend more than you earn for some time.
• Don’t expect sales to kick off immediately. This is a long-term project. It may take years until your marketing measures have gained some momentum.
• Connect with independent (local) booksellers. Talk to them, write to them – they might be more inclined to feature your book in their shop.
I had to learn some of this the hard way the past several months, so I’m happy to give advice to any new writers here.