Sorry it took me a minute to see this - I was answering all of
yellow's PMs.
There is no need to be a demi, although I think most mods were originally contributors because, as
yellow mentioned, they tend to look for some longevity, and people who have been here a while and help the site tend to become demis/contributors (not always, but often), so mods tend to be drawn from the demi pool.
The process is invitation-only.
I circumvented the standard route by making a $100,000 donation directly to
arn, but I can't promise that will always work. Even with the money, I had to put on a goat suit and record a dance for him. Something about having an embarrassing video of me so I never leak out how he got the funds. Ooops...
Anyway, your enthusiasm is well appreciated if perhaps a bit premature.