Go to Finder, click on your HD and see how much space you have free.
It should be about 8GB. If it's less than that, you need to do some clearing out.
The first thing to do is see what files you have on there and do some tidying. Is there anything you can archive off to CD/DVD/external drive rather than taking up space on your main drive.
Do a Spotlight search for files over 1GB just in case there are any odd system logs etc taking up space.
You might consider running Delocalizer to get rid of other languages on your applications. And if you don't use Garageband or iDVD, then clearing out the Garageband loops or iDVD themes, can save you about 2-3GB each