Installing new fonts is very easy in Mac OSX, Mac has a built-in application called “Font Book” for this purpose.
Download the font you want to install on your Mac. Lets intall Ubuntu font in Mac, which in not available in Mac by default.
Download the font and save it any location in your machine.
Open Go → Applications → Font Book
Click the plus icon and select the font file to install it
Tada! Font installed.. User installed fonts are displayed in User section in the Font Book Application.