Stuck with an old version of MacOS
From the looks of the details on EveryMac.com your Mac will take up to macOS 10.13.x (High Sierra).

Apple's current macOS version is 10.14.x so you are actually in pretty good shape as far as that goes. High Sierra is still getting security updates and most released software will work on the last 2 to 3 macOS versions. Many will work on versions earlier than that.

You should be able to sign into the App Store and download macOS High Sierra. here is a link that will open that up in iTunes, https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?mt=12

Once you have upgraded to High Sierra that will be the last major revision of macOS that is supported on your Mac. Depending on your needs and the apps that you use you may need to get yourself a newer Mac that supports macOS Mojave (10.14) and later. But IMHO you probably have a couple of years (at least) of life left in your current Mac.