This guide, once read in full, should answer all questions you might have about Enigmail and give you a thorough understanding of it. The Enigmail team strives to keep it up-to-date so that it contains the information about the latest version of Enigmail. In particular, you should always check the FAQ for an answer to your issues.
Should you experience problems or want to ask further questions, you can obtain support (provided by volunteers) from several places: there is a Forum and a mailing list/newsgroup dedicated to Enigmail. Just remember to ask smart questions.
If your question is not strictly related to Enigmail but rather to broad OpenPGP topics (e.g. signing, encryption, key management, trust...), you may also refer to the GnuPG documentation and support.