It's ready to use out of the box. It will do a software update if it needs to (install with your approval only) and does it regularly.
If you are using third party stuff now (firefox, t-bird, chrome, etc) you can download those and use them on the mac too. Do you need a multi-protocol IM program that does XMPP? (adium, worth it).
You should probably download a thing called growl (provides notifications from apps, etc, many apps will use it if it's installed, and you can control the notification levels, etc), and something called Perian. (the swiss army knife of codecs for quicktime). It's up to you if you are interested in 1passwd or not (a multi-platform password manager), oh, and you might want dropbox with a free dropbox account, certainly if you go with 1passwd. If you are already using another online backup program on your pc, you will probably want it on the mac to, if it's compatible, to make it easier to sync or bringover your data/media files. (I'm still using sugarsync on my PCs and also on my macs now, mainly because I was entrenched there.)
Other than that you will figure it out or ask us or other more mac-related boards.
get used to Command button instead of Control key for most things. (ie, copy/paste is command/C command/V on the mac.)
Make sure you get yourself an external usb drive (or some other method if you prefer, like network drive?) and set up time machine on it for your local backups. Get that going early on. Then learn how to unmount external drives
Oh, and enable/learn the multi-finger gestures on the trackpad, and use them (two finger tap for right click, and two finger tap and drag for scrolling are things you will get used to very quickly and try to do on your pc when you go back and then realize they don't work there.) The easiest way to learn the gestures is in the trackpad system preferences applet (aka, control panel for pc users.) They have little videos in there that demonstrate each gesture if you hover the pointer over the item.
-David