You can do this immediately after completing the first level, Port Royal. Just exit to the hub instead of continuing the story and once there, hold down Y, then scroll down to the bottom of the character select, choose any of the pre-made custom characters (you don't have to actually create one, as confirmed by me), and as soon you select the character and he loads, achievement unlocked. Enjoy your nice, shiny, new 15(G)!
Edit: Apparently, this may not work for everyone. While I got the achievement in this way, NamedCoUrTnEy19 has stated that it did not work. I know that I never once visited the character creator, and this was one of the first achievements I got. All I did was select Custom A in the main hub, but if this does not work for you, then you may have to wait until you are able to access Tia Dalma's shack through the left gold gate and enter the character creator, save one, and select it.