From what I can gather, your Mac came with Mountain Lion, OS 10.8.x. You can use a more recent version of the Mac OS on your machine, if you want. Did you purchase the machine brand new? Or was it used?
By the way, you state "I have not made a backup recently.". This would imply that you have an external device, correct? If so, 1) when did you last do the backup, 2) what software did you use for that, and 3) what is the make and model of the external device/drive?
Given that you have never done any disk cleanup/maintenance/repairs, it might be best to do the following:
1. Use Disk Utility to determiner the "health" of the internal drive inside your Mac (there are actually a couple of commercial products that are better than Disk Utility).
2. Assuming the drive is OK, use Disk Utility to Erase and Format the drive. (Depending on your answers to questions below, a backup might need to be made first of your current system).
3. Do a fresh, clean, "virgin" installation of a Mac OS.
Now, all of that needs to be accomplished in an "isolated" manner. But, another important question is whether or not you are using any third-party (ie, non-Appel) software. If you need to retain any such programs, and also if you need other documents. etc. from your internal drive, that will determine how to proceed.
So, if you can provide definitive answers to the following questions, we can better help you:
1. When did you last do your backup?
2. What software did you use for that?
3. What is the size, make and model of your external device? Is it partitioned?
4. Do you use any third party software? If so, which ones, exactly?
5. How much information/data do you need from your internal drive?