The Best Mac Cleaner Software of 2018: Unbiased Reviews. You can use the app to scan and clean system junk and old big files without limitations. Moreover, Dr. Cleaner Menu, which acts like a mini productivity app by showing a number of useful metrics that indicate how my Mac is performing in real-time.
The Mac OS is based on Unix and does a very good job of cleaning up after itself. I can't think of anything you'd really need unless you want to clear caches or something on a regular basis. In that case I would recommend the free Onyx utility. Read Thomas Reed's excellent article on the 'myth of the dirty Mac' here - http://www.reedcorner.net/the-myth-of-the-dirty-mac/.
As for anti-virus software, you really don't need it, either. However, if you're just paranoid and coming from the PC world, you might want to read Thomas' excellent discussion of Mac malware here - http://www.reedcorner.net/mmg/.
If you want to install some sort of free a/v software, I believe that Thomas recommends both ClamXav and Sophos. If you feel that you need them, download them.