Take a Liturgical Greek class, if your local university or Bible school offers it. Note that such a class will likely be entitled New Testament Greek or Koine Greek. Classes are usually the best way to learn the language, providing you both with a live instructor to help with difficulties and the motivation necessary to make progress.
Buy a good Greek grammar book. If you're taking a class, there will probably be required texts. Otherwise, find a book such as "Basics of Biblical Greek" by William D. Mounce, or "Learn to Read New Testament Greek" by David Alan Black. If you want a gentler introduction to the language before getting a full-blown grammar, try "Greek for the Rest of Us," also by William D. Mounce, first.
Use online resources to help you in your studies. NTGreek.org has a wide variety of resources, including reference sheets, a web grammar, and lists of recommended books. Teknia offers online classes and also sells books, DVDs and audio lectures. St. Jonah of Manchuria Orthodox Church's website has a massive collection of liturgical texts in Greek, allowing you to practice your skills reading the actual documents for which you are learning the language.