It depends on what would suit you. My BFF came up with my username as a joke and it just stuck.
If you ask me I agree with Lion_tamer, you can use your current user for your account, or you could just type random syllables until you find something that sticks.

Another option could be thinking of one word that describes you, for example it could be warrior, then translate the word to some other language and use that as your name?
Ex. Adelita = Female warrior in spanish

How about PinkBeltNinjaKitty, MagicalNinjaKitty, ThatRainbowDog, ThatRainbowNinja, PrismBeltedNinja, Prismacorn, Prismie, DoubleRainbows or TranscendentCat. Those are the best names I could think up for you. I hope you like them! :D

how about your planet minecraft name? RainbowNinja73?


or...you could use 'felonica' (or just type random letters but MAKE IT PRONOUNCABLE! it's ridiculous when people don't make it that way.)

FlxwerPetxl

L0velyB0nes

BoySlayer923593

Hmm.. Well, what sort of hobbies are you interested in?