Of course, the best thing to do is take a class by a professor who knows Greek.
But:
Interactive Learning Software has both the Greek and Hebrew programs and those would be my next best suggestion. The Greek program has the creative name of "Greek Tutor." I have both of them and they are really great. It has multiple exercises and it pronounces the words and letters for you too (one of the most important and helpful tools). You can be able to work through a greek passage in about a month or less, depending on how fast you get it and how much time you put into it. You should come out with a Greek II level of knowledge.
I will look online to see what else there is. The one posted by dellla looks pretty good.