Lightroom (Version 1.1 then) was where I started and the only place I'd tell anyone to start. From there, as Jeff said, Photoshop should be next, which, in turn can act as a host for the wonderful world of plug-ins, such as Nik and Topaz, to name a couple.
It all depends on your budget and how serious you are about learning the software and if you're not shooting in RAW, that's probably the first thing you'll want to tackle.
It's probably too overwhelming to do all at once, so starting with something like LR,4.1 which does soooo much more than it did when it first came out 5 1/2 years ago, might be all you'll need for quite a while.
I hope this helps,