Phonetic alphabets and fonts
The IPA (International Phonetic Alphabet) should be used as far as possible. The latest version of this is given in the Handbook of the International Phonetic Association (Cambridge: University Press, 1999): see also the International Phonetic Association website at http://www.arts.gla.ac.uk/IPA/ipa.html. The most common symbols used in Spanish can be seen by clicking here (.pdf format)..
In Spain, a system developed by the Revista de filología española has been used, with some idiosyncratic modifications and adaptations, in many textbooks, articles, monographs and linguistic atlases. It is in some ways more suited to the minutiae of phonetic variation within Spanish. Some of the more generally used symbols can be seen by clicking here (.pdf format).
North American and Latin American publications use a slightly different system again, the main variations being shown here.
Using phonetic symbols on computers and the internet is not easy. One of the advantages of the American symbols is that they are formed with diacritics from standard fonts, so many of them are contained within computer character sets (though not always within extended ASCII or HTML). The IPA symbols are not generally replicable from within standard character sets. To use IPA symbols within a word-processor, you need to install a special set of fonts. There are many available. The one I have used for some years on my handouts is SILDoulosIPA, which can be downloaded as part of a freeware package from http://www.sil.org/computing/fonts/encore-ipa.html.
It is difficult to represent phonetic symbols on web pages. If you use a font such as SILDoulosIPA, you need to be sure that the person reading the page also has it installed on their computer, otherwise it will be impossible to read, and so this method is not recommended. The most universal solution would seem to be to use UNICODE phonetic symbols, which can be decoded by most up-to-date browsers (for a guide on how to do this, see http://web.uvic.ca/ling/resources/ipa/charts/unicode_intro.htm). A proposal for representing phonetic symbols in plain text (which has the advantage of being a totally portable solution) is the SAMPA system, details of which can be seen at http://www.phon.ucl.ac.uk/home/sampa/home.htm; however, this has rarely been implemented for ordinary purposes. Another ASCII-based system is illustrated at http://www.antimoon.com/misc/phonetichelp.htm. For purposes of simple illustration, it is often easier to use a picture file (.gif for smaller pictures and .jpg for larger). The method I usually use is to export a word-processed document to a .pdf file with the relevant fonts embedded - but you need to purchase special software for this (unless you have OS10 or above on a Mac), such as the comprehensive Adobe Acrobat (see http://www.adobe.co.uk/) or the cheaper FinePrint PDF Factory (see http://www.fineprint.com/)