Well, you really started with the wrong album. I'm a huge Metallica fan and I think St. Anger is a gigantic, flaming pile of shit. The only two songs that are worth my while on that disc are St. Anger and Frantic. Everything else sucks. As it is, I'll throw some recommendations out from all their albums.
Kill 'Em All
Whiplash
Hit The Lights
Seek and Destroy
Ride The Lightning
Ride The Lightning
Fade To Black
Creeping Death
Trapped Under Ice
Master of Puppets
Master of Puppets
Welcome Home (Sanitarium)
Battery
...And Justice For All
One
Blackened
Dyer's Eve
Black Album
Nothing Else Matters
Wherever I May Roam
My Friend of Misery
Through The Never
Load
The House That Jack Built
Bleeding Me
The Outlaw Torn
King Nothing
ReLoad
The Unforgiven II
Attitude
Carpe Diem Baby
Fuel
Death Magnetic
All Nightmare Long
The Judas Kiss
That Was Just Your Life
My Apocalypse
I could have easily recommended more from each of their albums, but this list should be a pretty good start for you. And for what it's worth, you really can't go wrong with '80s Metallica. It's basically all good, with Master of Puppets probably being my favorite album from them as a whole. But I do feel sorry for you if you started out with St. Anger...album is...ew....I can't really put in to words how much I hate it as a Metallica fan. To me, it's basically an abortion of a CD, and something that should never have seen the light of day. Metallica would have been better off going 11 years without releasing an album between ReLoad and Death Magnetic than release St. Anger.
Also forgot to mention...checking out basically any single that they've released is also a good start, it's been pretty rare that they release a bad song to the radio. They may not be the best songs off their respective albums, but they're still good. Also, check out their live album, S&M, absolutely brilliant, and the addition of the orchestra makes for some of their classics to be even better.