The first thing you can personalize will be unlocked at level 10. You cannot get obtain this achievement until that level.
To do any editing, be in the multiplayer menu. You must complete the introductory session. After that, you will have access. Make sure you have pressed to access the Data Hub. Go to "Characters" and pick one that does not say "Not Customizable". You might have one based on your previous Assassin's Creed: Brotherhood experience. Remember, you should at least be a level 10. Go to main weapon and pick the one that is unlocked.