You can read most books in digital form. This also has the advantage that you can look up words with a built-in dictionary (for books, but not for manga). There are a number of options for buying digital books. The Resources for Starting to Read Japanese Content page has an overview of the most popular options, as well as a ton of other advice for getting into reading.
For BookWalker, they frequently offer free manga, and this way you can try out whether you like their service at all (like all the other options, you are locked in to their app or website, and this way you can see whether you get along with it well). You can check out the currently free offers in the The BookWalker Freebies Thread .
If you want to get an impression on the difficulty of a given book or manga, you can have a look at https://learnnatively.com/ (you should be aware that the levels are based off of a crowdsourced grading so in my experience they may be inaccurate at times). For books (not manga), you can also have a look at https://jpdb.io/ which calculates the difficulty off of the text (grammar and vocab) so this feels more reliable to me.
Other than that, like the others said, the best thing is just to get started! Pick something that you might be interested in and see how it goes. If it’s way too hard, you can always get back to it later. If it’s slightly too hard, you might want to tackle the challenge! If it’s just right, then enjoy the ride 