If you already have Mac OS X 10.6.8 you can easily update to either Lion (Mac OS X 10.7.5) or Mountain Lion (Mac OS X 10.8.2 currently.) Here is how:
Be sure your MacBook meets the minimum system requirements for the OS and the applications you want to run other than needing the OS upgraded. System requirements for Mountain Lion can be found here. Note that 2GB of RAM is the absolute bare minimum. More is better.
Also make sure that the applications you need to run on this machine will work under the new OS. As an example, PowerPC apps such as MS Office 2004 or earlier won't work under Lion or Mountain Lion. Check for all the major applications you use as well as things like drivers for your printers and other devices. Contacting the maker of these devices or looking on their web pages is a good way to do this.
Next, launch the Mac app store. In the Dock it looks like a blue circle icon with an A in it. Once open, do a search for Mountain Lion. You can purchase it here and download the purchase. It will then install.
If you decide you only want to go as high as Lion, you will need to contact Apple's sales department first to purchase a download code, as you cannot do that on the App store at this time. Once purchased, go to the App store and enter the code. Complete the download and install.
Once you have brought your operating system up to where you want, go to the app store to purchase the apps you want.
That's pretty much it. Best of luck.