If it is Greek Orthodox, it probably DOES have pews.
When you enter you will probably see some people selling candles and a candle stand of some sort. We generally put a dollar in the tray, and take a candle, and then light it.
If I were you, I would sit in the back and follow what those infront of you are doing. You do not have to make the sign of the cross every time someone else does unless you feel like it. Generally we do it when 'Glory to the Father, Son and the Holy Spirit is said'. Whenever the priest blesses the congregation, a small bow (in order to accept the blessing) is generally done, though in the Greek tradition, many will also cross themselves. When the priest comes down the aisle to cense, a small bow is again appropriate, and if you like, you can also cross yourself.
Also, there may be a liturgy book in the pew back infront of you. Try to follow if you can.
All in all, just relax. It is not very different from following along at a RC Mass, if you have been to one.
BTW--I am cradle Greek Orthodox. Have a blessed Divine Liturgy and let us know how it went.