How do I find the amount of free space on my hard drive?

Open System Monitor from Dash and select the Filesystems tab.

Or alternatively open a Terminal and type:

df -h

You can use baobab, or similar tools such as kdirstat or filelight, to see what files are using up your disk space.

Baobab is also called Disk Usage Analyzer on Ubuntu.

Here's a sample screenshot of baobab:

Below is a sample screenshot of filelight:

For a command line option, I prefer to use ncdu:

You can drill into sub-folders to get total relative disk usage on the sub-folders. It's turtles all the way down. More nifty than du -sh on remote machines.

gnome-system-monitor or df -h or lsblk

Other useful utilities are baobab.