From designing it and running it, I think I will be fine. I would use something like OS commerce. But Im just worried about the security aspect of it. I'm am not knowledgeable at all in security etc. Would updating all software and changing passwords be good enough for security? thanks
Yes, changeing pwd be good for security, but not enough. The most important thing for security is to enable SSL .
Usually, you'll just need to keep your software (oscommerce) up to date and have SSL installed for security. Of course, you also will want to have strong passwords for administrating the site.
Some of the solutions I have looked into at the pas was virtuemart addon for joomla and it was free last I checked. You also have zen cart. I am currently looking for wordpress addons and stumbled across this ... http://www.instinct.co.nz/e-commerce/ Good luck.