In order to use any font, TeX needs a metric file (TFM file). Several sets of metrics for common Adobe Type 1 fonts are available from the archives; for mechanisms for generating new ones, see metrics for PostScript fonts. You also need the fonts themselves; PostScript printers come with a set of fonts built in, but to extend your repertoire you usually need to buy from one of the many commercial font vendors (see, for example, “choice of fonts”).
If you use LaTeX2e, access to your printer's fonts is offered by the
PSNFSS package; the LaTeX3 project team declare that
PSNFSS is a “required” part of a LaTeX distribution, and
bug reports may be submitted via the
LaTeX bugs system.
PSNFSS gives you a set of packages for changing the default
roman, sans-serif and typewriter fonts; e.g., the
mathptmx package will set up Times
Roman as the main text font (and introduces mechanisms to
typeset mathematics using Times and various more-or-less
matching fonts), while package avant changes the sans-serif
family to AvantGarde, and courier changes the
typewriter font to Courier. To go with these
packages, you need the font metric files
and font description (.fd) files for each font family you
want to use. For convenience,
metrics for the “common 35” PostScript fonts found in most PostScript printers
are provided with PSNFSS, packaged as the “Laserwriter set”.
For older versions of LaTeX there are various schemes, of which the
simplest to use is probably the PSLaTeX macros distributed with
dvips.
For Plain TeX, you load whatever fonts you like; if the encoding of
the fonts is not the same as Computer Modern it will be up to you to
redefine various macros and accents, or you can use the font
re-encoding mechanisms available in many drivers and in
ps2pk and afm2tfm.
Some common problems encountered are discussed elsewhere (see problems with PS fonts).