Ok, let's assume you use Linux (Ubuntu) in my case and you need to have not just default system fonts but some additional ones. It's extremely easy to install it, just in few commands.
First, you have to create a new folder in home space:
# mkdir ~/.fonts
And then you just copy the new fonts into that folder and run the following command:
# sudo fc-cache -f -v
This will introduce your new fonts to the system. Re-run your applications to let them use freshly installed fonts. Enjoy!
PS. There are some other way how you can install fonts via package management software (apt-get, aptitude, synaptics, etc.) but this is a general way for Debian.