if you're not completely starting from scratch (as you say, you've done some tutorials) then go for it - you could have it fleshed out in a couple of hours.
the beauty of it is there are several existing cms available for you to dissect if you're up against the wall:
Radiant CMS
Rubricks - CMS on Rails
Railfrog - CMS Ridin’ on Rails / Ruby on Rails CMS
more for inspiration for solutions to particular problems than anything else - nothing stopping you hacking radiantcms to do what you want... though it's probably faster and easier to build your own
ideally invest in a couple of books:
Agile Web Development with Rails—Second Edition
Rails Recipes
the pdf's are like $20 - better for searching too
there are plugins for just about every functionality you'll require for a cms
there's a plethora of helpful people on the
mailing list & irc.freenode.net#rubyonrails if you're banging your head - it's certainly the most helpful and non-judgemental community i've been fortunate to be a part of
one caveat - file uploads. rails is single threaded so uploads are blocking; there are however solutions to this, the best being
merb - which is essentially threaded rails without the cruft (uses the same style of coding so it's not a paradigm jump); it's new, docs aren't comprehensive, there's less of the magic of rails so you'll have to think a bit, but i managed to pick it up enough in a couple of hours to build a little tumblelogging app in a few minutes so it's not difficult (if you have
some rails knowledge; most issues i had were with getting existing rails plugins to work which wouldn't be an issue just for uploads)
you'll also have to think about deployment options, which is a topic in itself - there are providers, depends on your budget; if it's bottom end you'll have to roll your own.
hth
edit: and a
merb tutorial
__________________meh.