Since hard drives fill up quickly with downloaded files, imported movies and pictures, a question I frequently get with people new to the Mac is how to check their hard drive space.
1. Control-Click or Right Click on your Macintosh HD (or any other hard drive).
2. Choose 'Get Info'.
3. Under the 'General' tab it will show you the drive capacity, free space, and how much you have used.
Some people get overly concerned about this, but I've found that for OS X to run speedily, you want to keep about 15-20GB free space. When you start getting low, then it might be time to archive some items to free up some space.