Q: hi, does anyone know what a blue ball....

You seem to be confusing "disk space available" with "RAM" (Random Access Memory) - two different critters

Cut down on open apps or add RAM (likely pretty cheap these days) as babowa advises

Well, that shows how much they know - not much. I'd question why he is still allowed to sell computers telling customers they do not need a backup.

You should back up your system and files - if there is a problem or hard drive problem, you may have to erase and reinstall - that's fine for the OS, but you would lose your files, movies, etc. So, have a backup. Time Machine is built in (Applications folder) - you will need an external hard drive for your backup; the size should be 2 - 3 times the size of your hard drive. For hard drives, you can check out OWC:

http://eshop.macsales.com/shop/firewire/EliteALmini/eSATA-FW800-FW400-USB

I've got 2 of those and 2 of these:

http://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB

You will need to format any new hard drive so it will work with your Mac - use Disk Utility (Applications > Utilities) to partition and format the drive. During that process, whatever software was on the drive will be erased. That is fine - external hard drives do not need any software or drivers to work; they do need to be formatted as Mac OS Extended (Journaled) with a GUID Partition scheme (under the Partition tab > Options).

After that,launch Time Machine and it will start backing up automatically; the first one will take quite a while. After that, it will just update.