You're partly answering your own question!
As you know, being a graphic designer requires you to learn specific software, get prepress (printing) technical knowledge and also learn how to handle different types of files (and so many issues too long to enumerate.)
If you want to do print layouts for cards you'll need to use design software that will give you all the freedom to do whatever you want to create and you'll need to learn how to use them. Being creative is often not enough, unfortunately.
What you are using online might be fun for now but it's possible you'll never be able to export that work into files that can be sent to a print shop. So for the long term, you might want to explore alternative ways to create layouts. Search for the Adobe products, and if you want free software, look for Gimp, Inkscape, Scribus, etc.
Some software come with banks of images or cliparts... but keep in mind that these graphics have been used already by many designers (by many I mean thousands or even millions)! So if you want to be original, you might need to learn how to create your own original artworks. You can always purchase "templates" online from stock picture sites but make sure to read the terms of use, especially if you plan to resell them. Even when you see free images or illustrations online, you need to make sure you read the license and terms.
Regarding the printing and the paper, that's something you'll need to investigate for yourself depending on the quality and quantity you want to get printed. Some people simply purchase a laser or inkjet printer and sell their own creations in small quantities, other go to a small print shop or photocopy center to get small runs of digital printing, and the most ambitious (or quality oriented or with higher budgets) will go to a real offset printer that can print hundreds or thousands of copies of your cards. All these different printing methods will require you to learn different skills to prepare your files in the best quality possible. Graphic design and doing print layouts isn't just about putting images together and making stuff "look good"! Depending on the printer you use, you won't be able to use certain type of paper. That's mainly a question of budget, and how much time and money you're willing to invest.
As you see, there's quite a few technical and legal sides to consider if you want to get in the business of creating layouts and selling them!
I encourage you to have a look at this question/post if you want to get started as a graphic designer:
Popular questions about Graphic Design - Start Here
