First, you need to grab a truetype font from somewhere. I'm not sure about where fonts are stored on redhat, or if you even have any truetype fonts installed, but they will probably be somewhere like here: /usr/X11R6/lib/X11/fonts/TTF/
If not, get one from somewhere else (download a TTF of dafont.com, or grab one from Windows (C:\system\fonts I think it is? not sure. but grab arial.ttf or something).
Then, you have a couple options. Two of them being:
just put the font into your ~/.mplayer/ directory, and rename it to subfont.ttf
or, make a symbolic link to it in that same directory. so if you have arial.ttf in /usr/X11R6/lib/X11/fonts/TTF/ then,
ln -s /usr/X11R6/lib/X11/fonts/TTF/arial.ttf subfont.ttf