Actually my character is a boy >_>
But as for how I got his skin so dark I went to the Island every day and had him wade in the water from 3PM-4PM until his skin got dark enough. The thing is though if you wanna keep your tan afterwards gonna need to keep coming back to the island between 3PM and 4PM and do the same thing for atleast 15 minutes, also don't close your 3DS while doing so and your character can't be wearing any hats or accessories.