Apple has made it tempting to upgrade your operating system. If you are running Snow Leopard 10.6.8 or higher, it is free and easy to go into the Mac App Store and just go for it. Not all Macs will run Mavericks 10.9. Apple has a list of qualified machines, some of which go back as far as 2007. I don't recommend upgrading those older computers. They will often become sluggish or have other issues after the upgrade. My rule of thumb: if it won't take 8GB of RAM, don't upgrade. Generally, that means nothing older than 2009 should be running Mavericks. There are a few models from 2008 that will take 8GB RAM, and several from 2009 that won't.
As you can guess, I do not recommend upgrading to Mavericks without 8GB of RAM (or more) installed. It is also very important to have a current backup. Technically, it would be better to have two backups, one Time Machine, and the other a bootable clone. But at least one of those is really a necessity. That is because upgrades can fail. And worse, they can stress your hard drive, which, if it is about to fail anyway, could die in the process. Admittedly, that is rare, but I have see this happen more with upgrades to Mavericks than any previous operating system.
It is alway empowering to tackle a major operating system upgrade on your own. And if you would like help with it, I can install RAM and check your backup (or set one up) before upgrading. Then I can show you how to make that shiny new operating system behave a bit more like the one you were used to.