Open your Zotero library window. Look at the top of the screen for a tab that says Zotero. Select it and a dropdown menu will appear. Select Preferences:
A window will open that has some choices represented as icons at the top. From this menu choose the Sync icon, and then choose the link that says Create Account:
You will be brought to a page where you can create a free account. Create a username and password there:
Once you have created an account, enter in your username and password associated with your account :
You are now set up to sync.
Go back to the main page of your Zotero library. On it, you should be able to see a green icon with circular arrows that looks like a recycling symbol on it:
If you click on it, it will begin to spin around, indicating that it is syncing.
This means Zotero is simultaneously uploading your local library to your cloud account, and downloading your cloud library to where you are locally. This way you will always have access to everything in your library wherever you log in.