Am I allowed to change my username
No you can not you can if you have not bean tagged or posted anything!
Go to your preferences, and check is their is a pencil next to your name. If their is, that means you can. If their is no pencil, that means you can't.
Keep in mind: you won't be able to in the future
But, you are new. So just click on your profile, click preferences, and you can probably see from then.
I changed the title so it makes a little more sense