Securing your Mac is easy... do nothing! 🙂 Seriously, the Mac is fairly secure to start with, and the weakest point in its security is you. The biggest risk right now is the recent outbreak of trojans. If, while surfing the web, you get alerts about how you have viruses and need to install some software, ignore it, and delete anything that gets downloaded. For more information, see:
http://www.reedcorner.net/news.php/?p=138
(* Disclaimer: links to my pages may give me compensation, and should not be taken as endorsement of my services by Apple.)
As far as updates go, just let Software Update do its thing.
For maintenance, again, do nothing. Well, with one exception: I generally recommend using Disk Utility to verify the disk (not verify permissions) before installing any system updates. If any problems are found, you should immediately reboot from your Mac OS X install disk and use Disk Utility (in the Utilities menu) from there to repair the hard drive.
There are many utilities that try to convince you to buy them to do regular "cleaning", and at least one (MacKeeper) uses some very unethical marketing practices. Don't fall for it. The kind of cleaning these utilities do is completely unnecessary and may even decrease the performance of your Mac or damage the system.