This can be really complex. I would advise buying a good Orthodox Day Calender (they make a great Christmas gift) for 2005. These are handy, and generally tell you what each day calls for. The reason it gets complex is that certain times feasts and fasts fall on the same day, and conflict. But, to keep it simple to start with:
We fast on Wednesday and Friday from meat, fish, dairy products, and alcohol.
(If receiving Communion) We fast from Midnight Saturday night from all food until after the Liturgy.
During the Year:
We fast during Lent from meat, fish, dairy products, oil, and alcohol. Your Church will cue you in when this begins each year, and it actually builds (you start with no meat for a week, etc.).
We fast before the Nativity for 40 days (i.e. from November 15th to December 25th).
We fast from August 1 to August 14th for the Dormition of the Theotokos.
We fast from the Sunday of All Saints (the Sunday after Pentecost) until the Sunday of Ss. Peter and Paul for the Holy Apostles.
Also, a few days, such as August 29th, the Beheading of St. John the Forerunner, and September 14th, the Elevation of the Cross, Great and Holy Friday, etc. we observe a Strict Fast, meaning we eat nothing (or as little as possible).
There are all kinds of exceptions and nuances to this, but these are the general rules, if you don't have a calender to help you. Also, for more specific guidelines for yourself, talk to your Priest. He (hopefully) knows you and can help you get into the mode of fasting without overtaxing yourself, and give you specific guidelines.