how can I get a surname?

[alienato]

how can I get a surname?

[Marigold Devin]

Welcome to Second Life Answers, alienato

You can add your voice to this JIRA requesting for the Last Names to be brought back

 https://jira.secondlife.com/browse/SVC-7125

[Cinnamon Lohner]

Your surname(last name) is Resident.  You will only need it for older viewers that require a last name.  Newer viewers use just the username.  If you would like to display a different name while in world, use the Display Name options as described HERE.

--Cinn

[Ansariel Hiller]

Right now at the moment: You can't! You can only set a display name, but that doesn't guarantee everyone will see it - either because they have display names disabled or they use a viewer that can't diplay them or display names are temporarily broken. However, there are plans to bring back last names.