Well, my take is that you shouldn't expect anything. Just relax, go the service and see for yourself what it's like. At least, that's what I did. I had read about Orthodoxy for a couple of years and I didn't come entirely unprepared, but when I first attended the Holy Liturgy on Sunday morning, it was still nothing like I had imagined it to be. Believe me, it was much, much better! You don't really know Orthodoxy until you've seen and witnessed it and have taken part in it.
If like me, you're not used to liturgical worship, you'll probably feel very awkward at first, but don't let that bother you. For instance, I saw people bowing, prostrating themselves, venerating the icons, crossing themselves during the services. I heard people reciting prayers in foreign languages from a prayer book, and virtually everything being sung.
Basically, just watch and learn. Try to concentrate on the words being sung or spoken. The Holy Liturgy is really profound, in my opinion. The vespers are basically evening prayers. Expect lots of psalms.
You'll probably have a stiff neck and painful feet the first couple of times, but I guarantee it's definitely worth it!