First you will need to complete a few missions to be rewarded with some little blue gems which actvas currency to buy your outfits etc.
Once you have enough, you can go to Outfits by pressing lithe menu button and selecting Outfits. I chose the Red Aloha dress from the Flower Outifts tan for 750 blue gems to unlock this achievement as I chose a male character. It’s pretty simple to follow, but I thought I’d add this guide to help out.