If you upload any csv to the Import Memberships wizard and then click through to the second screen, on the right you will see a drop down that will show you the list of fields that the Import can accept. That will include fields such as External ID, Email etc one of which will be required to match to the contact, as well as Membership Type, Membership Status, Membership Since, Membership Start Date.
If you have made custom Membership fields, then they should be shown in the drop down too.
Relationships can be imported via the Import Contacts wizard, but as Paul says, only a single Relationship Type at once. These live right at the bottom of the selector, ie after World Region etc. You choose the 'relationship type' that is to be created, then select the field that you have for the related contact you are making, eg Organisation Name if you are adding the Employer as part of importing the Employees. HTH