There are two approaches to phonetic fonts, Unicode and TIPA.
Unicode
TeX and LaTeX do not accept Unicode input. People at SIL have created XeTeX and XeLaTeX, however, which can accept Unicode input. XeTeX can be obtained as a MikTeX package, and also in various ways through the XeTeX web site. Note that for Unicode to be helpful, you'll need to use a Unicode font that has the appropriate symbols. Gentium (sans-serif) and Doulos SIL (similar to Times New Roman) are two options.
TIPA
TIPA is a LaTeX package that provides access to an extensive collection of phonetic fonts. To use the phonetic characters, you enter in a ASCII text that codes the symbols. The documentation contains a description of the system, and the associated commands.
A disadvantage of TIPA over Unicode is that the TIPA encoding is not used for any other application. If you enter your data in TIPA, it won't be helpful to you in non-LaTeX contexts (sending an email or writing a web page, for instance). On the other hand, entering data in Unicode format is far from trivial. TIPA provides a fairly straightforward and readable means to access special characters.