How the address is chosen
Your name is run together, lowercased, and stripped of anything that cannot
live in a web address. "Jean-Luc Picard" becomes jeanlucpicard. "O'Brien"
becomes obrien. Accents are folded down, so "René Müller" becomes
renemuller.
When it is taken
If somebody already has the address, a number is added: davebloom-2. That
includes addresses held by sites in the recycling bin, so a deleted site
keeps its address until it is destroyed for good.
Names you cannot have
A short list is held back for the platform itself: admin, www, api,
help, support and similar. Ask for one of those and you will be given the
numbered version instead.