Several ways, first off at the bottom of the finder window it should tell you how much free space you have.
Secondly, you can also select the hard drive, go to file and choose Get Info, and it will tell you how much space is used and how much is available.
To see what takes up a lot of space, I typically open the hard drive, change it to list view, then go to the View menu and choose "Show view options", then I check the box that says Calculate all sizes.
If you wait, it will show each folder and how much space each takes up. You can browse through and see that most will likely be in the users folder and possibly itunes or iphoto in many cases.