1. Just "go with the flow," so to speak. Observe what the other parishioners are doing and follow suit. Cross yourself when they cross themselves, prostrate and bow whenever they do, etc.
2. Most, if not all, of the Liturgy will probably be in Amharic/Ge'ez, so you might feel a little lost
3. Just dress modestly, and you should be fine. I think they'll realize your new to the Ethiopian Church and unfamiliar with her traditions. If you wear white clothes it might seem like your trying too hard to fit in.
4. Ethiopians probably more than any other OO group commune in EO Churches when they cannot find a parish of their own. I am not sure to what degree, if at all, they reciprocate this Eucharistic hospitality.
5. Ethiopians, like their Coptic brothers, remove their shoes when communing, some of them may even take their shoes off for the entirety of the Liturgy.
Hopefully others will chime in and elaborate on what I've said.