Agreed, do NOT get Norton antivirus (or any other variant), all it does is start to take up resources.
As far as getting viruses on a Mac go, it's next to impossible, unless you start using Microsoft Office (the Mac variant of course). There are a number of viruses out there which specifically target Microsoft Word, Excel and PowerPoint, and they will work just as well on Word:Mac as they do on the standard Word for PC! I'd do your best to stay Microsoft free, and try and use Apple's iWork. It can open & save Microsoft Office files, and unless you use some of the more advanced features of Microsoft Office (such as stupidly complex spreadsheets, weird Word formatting options or pointless PowerPoint animations), you'll be fine with iWork.
To be honest, I'd avoid using any kinda antivirus. The user at the other end is more likely to detect the virus than you if they're running a decent antivirus package (even if you are too), and if it slips through the antivirus on your machine, it'll do the same on theirs!
No need to Defrag or delete temporary internet files (Safari does this automatically, it limits the amount you can store and does a very smart job of doing so).
The only maintenance thing you might ever need to do is 'repair disk permissions' on your hard drive in the Disk Utility app, and really, you'll only need to do this if you start getting some weird glitches/issues. There is nothing you can do to your Mac to 'keep it going', it should keep going by itself. Just don't overload it with pointless dashboard Widgets or 'hidden' apps which sit up in the menu bar (you could only do this consciously).
Enjoy your Mac, they're a haven from PC land IMO. 😁