If you only want to buy one book, the simplest thing to do would be to sign up for a free account, then buy enough coins and purchase the book. Most light novels are 699 coins and most manga are 899 coins.
You can think of a coin as about 1 cent in US dollar currency.
However, if you want to get the best deal, I would recommend signing up for a Premium membership for one month. This adds complexity because you have to remember to cancel the membership at the end of the month if you do not wish to continue it. However each month of Premium includes 699 coins and a 15% discount on buying more coins. So you could sign up for a Premium membership, buy one book with the included coins, and be able to purchase more coins at a discount if you want to keep reading more volumes in a series.
A Premium membership would also let you read the catch-up series and the newly translated parts of ongoing series that are updated each week, so you can see if you like any of the other series beyond whichever series initially caught your interest. I have discovered several series that I enjoy this way.
J-Novel Club is an unusual publisher, in a good way. There's a bit of a learning curve to understand how it works, but I have been pleasantly surprised how thoughtful their design decisions have been, with regard to their business model and how they interact with readers.