You can see your available capacity by going to System Settings -> System Information. It's near the bottom, so you'll have to scroll down to see it.
The easiest way to clean up space on your PS3 is to remove some of the game data files (these are not the savegames, these are files that are cached on the hard drive to make the game run smoothly). These can be deleted from the Game Data Utility folder. If you decide later that you want to play that game again, it's not a problem -- the game will reinstall automatically.