Well, the best way to find opponents is through chat. If you'd like instructions, check out
http://fumbbl.com/chat.php.
There are no challenges (in the strict sense of the word) in the league at the moment. Right now, you simply check on the chat room and see who wants to play. Hopefully, you'll even find someone with a rookie team.
There's an extensive FAQ available that is well worth reading.
http://fumbbl.com/FUMBBL.php?page=help& ... ingstarted
Also be sure to check out the Rules page and eventually the Divisions page (the tabs at the top of the page). It's really not a lot, and PBS keeps telling us that reading is fundamental, right?
If you want a hand getting used to the BBowl client, just ask around for a practice game (if you don't want it to count) or just someone willing to play a rookie through his first match (if you want to start racking up those SPPS and $$ right away). I'd be happy to, if you catch me.
Also, the forums are quite active recently. Give 'em a look.