Mac computers need 10% free of their hard drive to really run well.
There are are two ways to see how much space you have free.
1. Open a new Finder window. This could be your home folder, your applications, etc. On the bottom of that window it will tell you how much free space you have.
If that bar isn't showing for you on your keyboard hit the CMD key and the / key (CMD + /) and it will show
2. Another way to see your free space is to Get Info on the Macintosh HD. If you are showing your HD on your desktop, click on it and hit CMD + I (I for info)