I have trouble mapping over basic Account and Contact fields. I use the wizard to test a small batch of Account/Contacts but not all the fields are listed when I have to map over half the fields. Also, how does a Salesforce ID get assigned? I tried to add my old customer ID but that won't map over. I did use the correct Wizard header, selected Upload and Account/Contacts and Add New Records. Thanks.
With the NPSP, if you're trying to import a new Contact for the org, the ContactId from another Org and AccountId from another org is (ditto for any Salesforce Org) is irrelevant. Each org will assign their own unique Id's to a Contact and to an Account. What's more important to know about the NPSP is that the paradigm is totally different and in fact opposite from what one is used to doing in a standard EE Org.
In the NPSP that's using the Householding model, one creates or imports a Contact first. Salesforce then creates an ID for the Contact and an Account for it along with a Household for the primary Contact/Account of the Household. Once you've done that, any other contacts that are related that reside in the same Household should be created from the Household. Salesforce will then create an Account for each new Contact and associate that Account/Contact with the Household. Those additional contacts however will not be the primary Contact/Account holder for the Household.
At some point for example, children might go off to college, but still be associated with the same Household. They can have their own address while still associated with that Household. Alternatively, the Org can decide to move them to a Household of their own. That's some of the flexibility built into the Household model.
If importing multiple related contacts into the same household, each needs to have their own unique email address. The contacts need to be imported into the import object you can learn more out about by reading the documentation on the Salesforce.org site. To learn more about how the Household model works, I highly recommend the Manage Fundraising for Nonprofits Trail on Trailhead