Most lab tests require fasting for 8 hours beforehand (water is fine), unless your clinician has told you otherwise. Check your specific test's instructions for any other requirements.
Stopping supplements 2 days prior is also recommended for most lab tests, unless your clinician has advised against it.
A few other things to keep in mind:
Bring your requisition, available in the Documents section of your portal. You can print it out or screenshot it.
Wear comfortable clothing that makes it easy to access your arm.
Bring water.
And once it's done, treat yourself to something sweet, you earned it!