It's not really necessary on a Mac, or Windows as much as people think really.

The number of actively spreading viruses/trojans/worms/whathaveyou for OS X is miniscule at best. There is a malware remover built into OS X and it operates silently in the background.

Companies will be happy to sell you an AV program for OS X, but as I'm fond of saying: If there's someone stupid enough to buy it, there's someone smart enough to sell it.

And those system optimizing programs are a complete scam, Windows, Mac, and anything else. At least since XP, Windows has been pretty good at taking care of itself, and OS X is basically a modified FreeBSD, which can ultimately trace its roots back to the original BSD Unix, which itself is just a reimplementation of the original Unix. So, we're talking around 30 some odd years worth of collective experience, virtually all of that aimed at being a 24/7/365 OS, where the only time you'd EVER shut down was hardware related. There are a lot of maintenance programs which are run automatically at the Unix layer of OS X, and you don't need to do a thing. Microsoft has been taking a lesson on this too, and Windows will generally take care of itself just fine. All Windows needs is for you to keep out of its way and not do stupid things that invite trouble, like using Internet Explorer, downloading dodgy crap from suspect sources, and thinking that your intermediate beginner level of knowledge is greater than the collective sum of all the Windows developers.

Computers these days tend to be able to take care of themselves in large part. You don't need to do anything but let the built-in programs do their thing and keep out of the way. Your job is getting rid of all these outmoded ideas about needing to maintain or optimize everything.