O2 sensor? What? Where?

[Elliott Potter <epotter@abraxis.com>]

I seem to be getting HORRIBLE milage on my '93 90CS.  Also, the engine
seems to be laboring a little bit, especially around 2700-3500rpm.  My
two ideas for this were clogged catalytic converter or bad O2 sensor. 
Well, I'm a little new to this and have never really done any real work
on my car (except installing Dynamat, but that really doesn't count...),
but I decided, 'what the heck; I'll just have a little look at the O2
sensor myself.  Maybe a hose is cracked or something.' ...
...
...
OK, I give up...
WHERE THE *$&#*&@*&#%*&$ IS IT??!! (screaming and cursing intentional)

Am I even on the right track anyway?  If I didn't make it clear before,
I've never done any work on this car, and in fact haven't done any work
on _any_ car since, well, does anyone remember the early 70's Fiats? 
The last engine work I ever did on a car was to replace the engine on
one of those with a catalog order 2 cylinder lawn mower engine...

_Any_ help would be appreciated.
Well, OK, any help except being told to go to the dealer?  I'd really
like to stop going to the dealer if that's OK;
One has to learn sometime...

Also, please reply by e-mail, as I'm still 53 digests behind...

TIA

Elliott