When you upload your contact file there are different columns as you mentioned, but it is not required to enter in a contact's first and last name. When creating an email you have the option to include your contact's first or last name (if it's in the system) by using a Greeting Tag, or you can have other information populate by using the Contact Details option. If you choose to insert a Greeting Tag then you also have an option to include a default greeting for contact's whose name you do not have. It will look like this when creating the email in your account:
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