Firsst follow the steps Shadow Ice told you, keep the Brushes window in a minimized window for later. Now download a Brush deviantart, press "Save", now press Open Folder, find the higlighter Folder in the new window. Double Click it, Double Click on the font (Not a typo, you'll see what I mean. =]), now Drag all the new font into your Brushes Folder, open GIMP, and they should be there. =]
Tip: When at Deviantart search for "GIMP Brushes".
Hope this helps SheimiGiratina! =D
~SotS