After finishing the brief tutorial you will be free to do what you want. You should see an orange button with 3 ^ icons on the bottom bar of your screen. Click this, then click "More", then click "Account". Now you will be at a screen where you see your automatically generated name but luckily the game gives you a name change token to begin. Simply click on the text, delete it and type the name you want and confirm. The achievement will then pop!