You need Virtual Currency to buy tattoos from the in-game store. You can earn virtual currency in the game itself. After every game, you will be rewarded with some amount of this virtual Currency or you can spend real money to buy. Once you have enough VC, search for 2K Store in the main menu then Everything Else category, there you can buy tattoos of your choice. Applying tattoo is an easy task. Simply go to the MyPlayer Appearance, there select Player Customization. After that select Choose Tattoos then Search for the tattoo you want to apply and apply it on your Player's body part.