I’d definitely check out Book Walker, it’s fairly straightforward to use. On the main page - click on the padlock and the word ログイン(login):
On the next screen you can click on the globe icon and change the language to English for the registration process. The same icon comes up when you go to checkout so you can do the checkout process in English.
To put something in your basket click the green button - カートに入れる (put in cart). You can also click on the book cover to go to a preview of the book.
Click on the shopping cart at the top right to go to your cart and complete the purchase.
If you’re feeling nervous you can download one of the free books that say 0 yen next to them on the home page to have a play. E.g. this one is free at the moment: クレヨンしんちゃん. Good luck!