Welcome and most of all enjoy your new Mac. Spend some time playing with it and get to know your new baby.
As on the virus thing. DONT WORRY ABOUT IT.
As it stands today there is know known viri out there to affect your Mac. Have a read of
The official Mac antivirus and firewall FAQ
Its a good read and written 5yrs ago but still pertains to today. You will only need AV if you want to run windows on your Mac and then only need it on the windows side.
Just use the search function at the top of the forum to find out about everyones favourite Apps as there is a plethora of them out there

but ill put in a few
Adium : Multi client chat
Handbrake : Video and audio converter
VLC : can watch and listen to basically any video or audio codec
But there is a lot on your Mac as well that people tend to over look so as i said play with it and see what you have

TM
EDIT
Wiki : list of Mac Apps