Most people arrive at CiviCRM with at least a little bit of data from somewhere else, even if it's just an Excel spreadsheet. Without covering the limitless possibilities of the types of data you might import, this video shows the best practices for importing contacts (Individuals, Households, Organizations, etc) into CiviCRM along with their basic information: address, phone, email and so on.
I'll discuss dedupe rules, import settings, data cleaning tips, building relationships when you import... and the best ways to avoid making a mess as you transition to CiviCRM.