By clicking on the picture of the item you get more details on the item. Not only will you find crucial information there (for instance: size, stock, material) you will also find an "Add to Cart" button, or a big "Buy it Now" button.
By clicking the Add to Cart button, you will add it to the cart. A so-called drawer will open from the right side of your screen, showing the contents of your cart. You will be able to collect multiple items this way. There is also a cart icon in the top right of the website. If it has a red dot, that means you have something in there.
If you click Buy it Now, you will be directly guided to the checkout. This way you can immediately purchase one thing. You'll be able to review your order before hitting the confirmation button.