Hi @Laly Lichtenfeld​ I recommend you take a look at a few resources:

• Review the Get Started: Salesforce Fundamentals for Nonprofits Overview & Webinar series
  • You can also view Webinar Recordings on the Salesforce.org YouTube channel HERE
  • I recommend watching the recordings by starting at #1, then working up to #4, which is about data import
• It will depend how much data you have, + @Data Quality & Management​ and @Nonprofit Get Started Hub​ who can help advise!