It may sound trivial but it is absolutely not. You should always, without exception, have both a first name and last name in every contact. When you have it, use a middle initial too, to distinguish between two people with the same name.
If all you have is the first name of John, and you have no last name, you should just put John, or maybe TBD (To Be Determined) in the last name field too. This enables you to order your contacts by last name, and see for whom you need a last name.
Why? If you do not, you will eventually end up with a long string of contacts in your database with only first names when you order your database by last name. If you ever transfer your database to another one, it ends up being a mess. If you ever do a mailing or e-mailing which requires both the first and the last name it looks very unprofessional and “canned”.
Many people have a company as a contact and only have a first name. Again, put something in the last name field too, even if it is just repeating the first name field. Best practice – First Name = ABC, Last Name = Title Company.