Before we sign up for a particular site, we always had ideal usernames in our minds. You can always give yourself a cool surname. People don't always matter the surname you choose as long it's appropriately good.
I'll start mine. Reasons for choosing Pinkgirl234 as my username:
Pink-girl-234
Pink: Pink is my favorite color because to me it's beautiful. It can also be derived from Pinkie Pie since she is pure pink.
girl: I am a girl.
234: Just a set of numbers I chose to place on my username.