Once you have created an account, you can return to the desktop application and sign in. To do this, select Preferences under the toolbar tab Edit (on a Mac, select Preferences under the Zotero tab on the menu bar).
Next click on the Sync tab of the Preferences window. Enter in your username and password to your account and select both the Sync Automatically option and Sync Full-Text Content option.
Zotero gives all users 300MB of storage for free and additional storage can be purchased on their website
To access the stand-alone client (the desktop app) on your computer, open it from the programs menu. If you have not yet added the Zotero connector for your browser, you can do so from the Download Webpage.
Borrowed from UNC University LibGuide https://guides.lib.unc.edu/zotero/getting-started#s-lg-box-126659
The above image displays the layout of the Zotero pane. From left to the right the buttons are for: