Update to 3.0.1 (if you havent already)
Download redsn0w for mac.
download ipsw 3.0 for your 3GS ( i usually use
http://www.felixbruns.de/iPod/firmware/ )
run redsn0w and follow the instructions.
Easiest thing in the world to do.
Ermm, cydia, basically its a program that is installed onto your iphone, and theres loads of stuff on there, themes, applications, ringtones etc etc etc. and you simply go on it, and download what you like lol easy as that!
Need any more help or if something does wrong give us a shout
🙂