Not a problem at all if you haven't, just might be a reason for slowing down if you had.
The simplest way to load a font is to either double click the font file (that you downloaded, for instance) and it will load it in "Font Book" or to open "Font Book" in Applications and click File>Add Fonts and select the file from there.
Font Book is a pretty awful font management program if you have hundreds of fonts, but by the sounds of it you don't, so it should be fine.