The online shop (called iShop) is available at any time once in game. Inside, you can find various gears or costumes to change your style in something cute, evil, or impressive!
There's also items who can give you more experience or a higher drop rate when you kill monsters!

First of all, let's see how to open the iShop.
In the top right corner of your screen, there's a little icon with a coin, click on it:

A new window will open, you're now in the iShop.
Buttons on the left are categories, you can scroll pages by using the arrows at the bottom.
Now, to get Karat, cick on the upper right button:

It'll open your internet browser with the Ragnarok Online website.
You now have to login:

Once it's done, you can choose the amount of Karat you want:

Wait a few seconds and a Steam interface will appear, then click on the green button at the bottom.
(Please note that this page will of course be in the language you speak)

On the next page, Steam will ask you to add credit on your Steam account, you can choose what you want or select the default value.
Be careful, this is not the amount of Karat you'll have but the amount of credit you'll add to your STEAM ACCOUNT!

Proceed to the next page where you'll have to agreed with terms of agreements and click on the green button.

Once the purchase is validated, you can go back in game (relog if you were still in game while purchasing), open the iShop again and your amount of Karat will be displayed at the bottom.
You can now buy items with your Karat!!