An e-commerce website that is easy to update

I have built simple websites and uploaded them using dreamweaver before but do not feel confident in developing a full e-commerce website. I would like some sort of content management system that is easy to update the products. Obviously the system has to be very secure as I am taking payments. The website will be similar to this one: http://www.theglittertree.com/. I have posted a job for a logo and graphic that will be included at a later date. Ideally I would like the website produced within the next 3 months.