Before creating accounts and profiles all over the internet, make sure your best username is available. Pick a good one and claim it all over the internet. Protect your name or company name before some one else takes it. Your username should idealy include your specific topic.
If at all possible try to make your username speak for itselves. If you have a car-parts website, your username should idealy be “CarParts” or something like that. Or you can just use your domain name perhaps.