Upgrade Zencart

I need an upgrade and update on my Zencart website.([login to view URL]) I would ideally like to find someone that would monitor my site and do this regularly, but I am a very small business and wouldn't be able to afford much. So I would like someone who could teach me the basics of how to do upgrades/updats myself myself. I also have some security issues. I have been hacked twice - once my orders were hijacked (though I didn't actually lose any money, nor were my customers' CC#'s compromised) and once I was used to send spam. I am a very small business and only get a few orders a week, so I can't afford anything expensive - I would like some suggestions as to what would be best for me.