Hi,
f you feel this task will be too much for you and your organization to complete please reach out to your Account Executive to get you in contact with one of our Salesforce Consultants that can accomplish the Data Migration for you. This article is under the assumption that you have all the vital Meta Data existing in your New Org.
We recommend that you watch the video, How to Import Data into Salesforce to become familiar with what will be required to import all your Data from an Old Org into a New Org.
To import records from one organization into another, you'll need to import the records in a particular order to maintain their relationships. Their relationships are defined by their Salesforce IDs, not their names, so you'll need to match IDs and Names up. You can do this with a Vlookup.
If a VLookup isn't something you're comfortable with, you can use existing Record IDs as External IDs on most Objects.
If you want to only import Accounts, Contacts, Leads, Solutions or any of your custom object records, you have the option to use the Data Import Wizard.
In the event that you need to insert all other objects not listed above, and you're on the Enterprise, Performance, or Unlimited editions, you'll need to use the Data Loader.
Use the VLookup functionality to map the name of the Account from the Account Record into the Contact records based on the displayed Account ID. When performing the import, you'll be able to perform the imports in any order you desire, though the Accounts and Contacts will need to be imported jointly.
It is best to import or recreate your Users into your New Org once you have your Meta Data in order. Remember that you need to alter the Usernames in the Old Org so your Users can use their preferred Username in the New Org.
Hope this helps.