If you use more than one computer, Zotero's sync feature can keep your library up to date on all devices. All devices must be running the same version of Zotero (your computer will typically check for any updates to versions automatically).
PLEASE NOTE: Syncing does not work well with multiple users on the same device. So while you can sync libraries across multiple devices, like a work laptop and a home desktop computer, all devices should be your own personal devices (or if they're shared, you should be the only Zotero user tied to the login information used). For example, if you are a student worker in the library using a shared login to access the computer, do not attempt to download and store citations in Zotero on that device. Otherwise, citations can become mixed between libraries.
First, set up a free Zotero.org user account. Then:
Repeat this configuration on each of your computers. Any updates you make on one of your computers will be reflected on the others. This even works to synchronize your library among Windows, Mac and Linux computers.
For more details and help troubleshooting sync problems, check the Zotero site.
Before creating a bibliography, you will always be prompted to choose an output style. However, if there's one style that you primarily use, you can set that as your default style.
To set a default style, go to the Edit > Preferences screen, then click on the Export tab. Click on the drop-down menu under "Default Format," locate your style, and press OK.
Again, you will always have the chance to use a different style upon the creation of every new bibliography, but setting a default simply saves you a few seconds of scrolling through the citation format list by having your default style pre-selected for you.