First copy your font file - it's a .ttf, presumably - to your Documents folder, using the file manager.
Now press Alt+F2 to call up the Run Program dialog. Type 'Terminal' (without the quote marks) and hit Run.
When you have the Terminal window open in front of you, type 'sudo cp /home/user/Documents/font_name.ttf /usr/share/fonts/font_name.ttf' (again minus the quote marks).
Any apps you want to use the new font in should be quitted and then started again to recognise the new font.
See also Ten Tips for a new Aspire One: