Well at a minimum you need Mics and a Recorder. What is your budget?
Start with the Recorder (since that is easier to decide on)
Minidisc $
DAT $$$
HardDrive/Flash based recording. $$-$$$$
Newer Minidiscs can record in uncompressed formats and start off fairly cheap ($150)
DAT isn't really being made anymore, but you can get one from around $300 and up.
HD/Flash-based recorders start with the Nomad Jukebox 3, ($130 used) and move up.
Mics? You can spend anywhere from $75 to THOUSANDS of $$$.
Do you tend to be close to the speakers and "Stacks"? or further away? That makes a difference in the kind of mics you MAY want. Listen to different boots and find out what you like.
A decent starter package would be a JB3 with some Core Sound Binaries Mics. (I'm not a big fan of the mics, but they can pull a decent enough recording to share/listen to) You could get started with that for around $200.
My setup (3/28/05, 4/15/05 plus several other shows out there) ran about $1300 including the cables I needed and a cheap Digital in soundcard (something you don't need with the HD/Flash based recorders)
Dat = $750
Mics/Battery Box = $350
Cables and Soundcard $150