This is a large topic, which is why this answer is long (and growing as I make edits). I have thus appropriately bolded important parts and beginnings of topics to make this content more navigable.
Gigabytes, or GB, are the standard unit of measurement of harddrive space nowadays (there are others, but we will ignore them for now).
The general rule of thumb for computers is to keep 10% of the hard drive free, or else your computer may start to slow down. In this case, it's apples to apples - GB is a unit, you can simply divide the numbers like so:
Available / Capacity * 100 = % free
In your case:
13.89 / 74.21 * 100 = 18.7% free
You have more than enough space free, you don't need to worry about cleaning up files. However, if you would like to clean up the files, here is how you figure out a file or folder's size:
In Finder, click once on the file or folder so that it is selected. Then press cmd-i (short for "Get Info") and a window will pop up with a bunch of information about the item you selected. To the right of the name at the top you should find the size of the file, which will be shown in KB, MB, or GB (as explained below what those are). For folders, it may show "--" at first while the computer calculates the size. Be patient and it will show up eventually.
Here are the rough approximations of size, should you see other units (ABCellars' answer has the full and exact explanation, so I'm making this as short as possible):
Byte - literally a "bite-sized" chunk for the computer. Almost never used. OS X won't even let you create a file of this size! It's that small.
Kilobyte, or KB - around a thousand bytes (as the SI prefix kilo- tells you). A very small amount.
Megabyte, or MB - around a thousand kilobytes, or a million bytes (again, the SI prefix gives it away). Audio files are generally on the order of megabytes, with an MP3 song being somewhere between 3 and 10 in general, depending on length. A medium sized file will be in megabytes. A file is not a "disk hog" if it is several hundred megabytes.
Gigabyte, or GB - same deal, about a thousand megabytes, or a billion bytes. These are seriously big files. My hard drive is about 830 GB, and I don't anticipate on getting it anywhere near full anytime soon.
Some examples of files on each scale:
KB: text files, small sound files, small pictures
MB: Songs, high quality pictures (higher than cell phone quality, in general), very large text files, a Powerpoint presentation, short videos. A CD can contain up to 700 MB of data.
GB: Movies, large computer games, large applications. A standard, single-layer DVD, such as one you might find a movie on, fits 4.7 GB of data.
How to free up some easy space on your hard drive
As Mayer said, check for duplicate songs in iTunes. If you know how, you also may want to convert your songs to a more compact format, although doing so is tricky and may result in a loss of sound quality. Should you wish to do so, there are plenty of articles on the internet weighing the pros and cons of this - it really is a topic unto itself.
Also as Mayer said, burning files to DVD is another easy way to free up disk space, however burning DVDs makes them much more inaccessible (for viewing and editing). I keep space on an external hard drive specifically for large files (home movies and such) so that I can access and modify them at will. However, in a pinch a DVD will do the job.
If you have an iPod or other iDevice, you can use extra space on them as a hard drive if you allow the option in iTunes. On the main device management page of iTunes, you can check the "enable disk use" button and instantly use your iDevice as an external hard drive.
I would also recommend a spring cleaning, since it is that time of year. Just going through all the folders on your hard drive and selectively deleting content can add up to some significant savings. (pro tip: pressing cmd-delete with a file selected sends that file to trash - no need to drag and drop) There are also programs out there for Mac that will clean out hidden files that the system no longer needs, such as library and log files.