For launch, you will be able to create spells, magic items, and monsters. You can navigate to the creation forms from the Spells, Items, and Monsters buttons in the main nav, from the profile menu, or by visiting dndbeyond.com/homebrew/creations/create-[spell], [magic-item], [monster].
The basic information for these three content types is all that is required to display a magic item for your character to equip. However, to take advantage of the more advanced features on DDB, you can provide additional information and add various modifiers. This will allow the magic item to be included in automated calculations (such as adding a +3 magic bonus to plate armor - the resulting bonus to AC from the armor would be 21).
The process for supplying the advanced information for spells, magic items, and monsters is, well, advanced. It's not for the faint of heart. Over time, we will listen to feedback and see if there are ways to improve the experience, but at its core, it's not intended for beginning players and will take a few times for a veteran to feel like a master.
A great place to start figuring out how the various modifiers work is to create a spell, magic item, or monster from a template of existing content.
Using this method, you can see how we actually parsed the data for those elements.
To view all of your created content, visit the My Homebrew Creations page at dndbeyond.com/homebrew/creations.