Don't install any anti virus software on a mac! The cure is worse than the (non-existent) disease.
But, yes, be VERY careful about typing your password in. Know why you're doing it. A bunch of people installed free PSE and iWork from bit torrent and they contained malware. This of course is not a virus, but rather is a TROJAN, although all the virus protection companies were quick to proclaim that macs now suffered from viruses, just like PC's.
Anyway, I'd recommend the "OS X Missing Manual" book. It's quite good and well worth the money.
And, in general, I'd say that do NOT do any "maintenance" type stuff that you used to do on a PC, like running a defrag program or trying to go into the file structure of programs and trying to empty folders of temp files and such like. And when in doubt, try to the simplest, most straightforward way to do something first before resorting to more complicated methods. Like, for example, DO NOT automatically install drivers for things. In particular, do NOT install Logitech Control Center. It's a nightmare, according to reports I've read. If you buy a logitech mouse, just see how it works with the native OS X drivers. If you need more functionality, try Steermouse or USB overdrive programs.
And, do check out a mac forum, like
MacRumors.
And start using Expose right away, if you haven't already. I think it's the F3 key on macbooks (just like my MBP).
And, also, try the native OS X programs before buying special replacements. Like Mail. OS X Mail is one of my favorite email clients. And iPhoto has gotten quite good, starting with '08 and onward. It will even handle RAW formats now.
OK. Cutting myself off.