It really depends on your book and your goals. If you just want to get your book out there and make it available to as many people as possible, then you can start by listing it on online retailers and marketplaces like Amazon and Goodreads. You can also create a website or blog for your book, and use social media to promote it.
If you want to generate more interest and get people talking about your book, then you'll need to be more proactive. You can start by writing guest posts on other blogs, doing interviews and podcasts, or even organizing your own events. It's important to get creative and think outside the box to really make your book stand out.
There are a lot of ways to market a book online, so it really depends on what you're looking to achieve. Take some time to brainstorm and come up with a plan that will work best for you and your book.