Without a design, it is impossible for you to start 3D printing. You could design a 3D model yourself, but this can be rather difficult. Instead, it might be better to use a website to download designs that have been created by other 3D printing enthusiasts. This is especially better for beginners. There are websites that offer free 3D designs, while others offer paid ones. You could use several 3D printing services and design websites, but the three below are the most common ones.
If you love 3D printing you need to know and use Shapetizer. It is a great website if you want to download 3D models. You will find files from essentially all popular categories, including jewelry you can 3D print. Besides that, on Shapetizer people also upload models of video game characters and stuff like that. Using the perks offered by the site can only be done by registering with Shapetizer. After you have done this, you can experience all its benefits. Some STL files are free, while others need to be paid for. 3D printer files streaming is possible as well.
Thingiverse is quite possibly the most well-known site for this purpose. Many users have uploaded their 3D model on the website so others can download and print it as well. Thingiverse is extremely simple to manage and has so many great 3D designs that you can download for free. On top of that, the community is already quite old. Because of that, thousands of models have been listed on Thingiverse for you to use. However, the database just keeps on growing. Every day more designs are added to the website, which you could potentially do as well.
Many different 3D designs have been stored on a website called YouMagine. This platform is managed by a company called Ultimaker. You may know this business, since it is quite a popular 3D printer manufacturer. On top of that, Ultimaker is responsible for developing the often used Cura slicing software. YouMagine has been on the Internet since 2013, so its database of 3D models is very broad and impressive nowadays. At the moment you can find over 16 thousand designs! These STL files are free and can be downloaded in order for you to print them with a FDM or FFF 3D printer.