Username questions? 10 points!?

[fkdgjfj]

Okay, I have a question about making usernames. Can I have anything as my username? Let's say I wanted "Gggg3434" as my username right, but it was similar to someone else, is that still okay too have? Another thing is, whenever I try to come up with a new username I end up making something else like once a week. Sometimes I can't help myself and just another one if the new one I made is better or whatever. How can I stop making usernames as well? So that's two questions that I want to know the answers too. Thank you to anybody that helps.