I Think The Alternator Is Broken?

My Cadillac el dorado had battery troubles about 6 months
ago, replaced the battery and everything seemed fine until a
few weeks ago it began dying right after it started. If I held
my foot on the gas it would stay on but then die once I let
up. It would occasionally work to hold down the gas and
then quickly shift in to gear so since I'm painfully low on
cash I limped her along till last week. While turning in to a
gas station I noticed my radio started dying. I scrolled
through my dash info and watched my battery fluctuate
and start to lose volts while the car was running... until it
stopped doing even that. I jumped it and got it home but
she doesn't seem to hold a charge while running, and since
the battery is new I figure it's the alternator.

NOW, because I'm poor and also only BARELY capable with
car repair/maintenance I'm not sure what to do next... I've
never had an actual mechanic fix my car, I've always had
junkers that I fixed up what little I could with googling and
YouTube videos until they died and then repeat.

I don't know how to not get screwed since I'm not SURE it's
the alternator I wouldn't really know what fixes and costs to
agree to. I'm a young lady, so sometimes mechanics
assume I'm clueless, I'm not entirely, but unfortunately I'm
only as not clueless as what I've learned from my trial and
error mechanical engineering. I just want to know what to
do next, is an alternator easy enough to install I could skip
the middle man? I'm up for some elbow grease but if it's
incredibly technical I'd rather try my luck with a mechanic.
Help!