Hi, KAYBEEU.
Thank you for the question! I understand you want to upgrade your iMac to an operating system that will allow you to utilize the Mac App Store. From the system information that you provided, you can at least run Snow Leopard and it is available for purchase via the link below.
Mac OS X 10.6 Snow Leopard
Mac OS X v10.6 Snow Leopard - Technical Specifications
I would need more information on your hardware to see if you are able to run El Capitan. However, take a look at the link below as it will show you how to check to see if your system will support El Capitan.
Upgrade to OS X El Capitan.
If you are unable to upgrade to El Capitan, there may be other operating systems that your iMac will support. Go through the link below and find out which model iMac you own as I may be able to provide additional solutions with this information.
Identify the model of your iMac
Thank you for visiting Apple Support Communities.
Cheers