If you are an experienced Modern American (Western) Square Dancer, you have a bit of a head start on English Country Dance.
It is not necessary to prepare for the class in any way, but if you are interested, I wrote an article about English Country Dance for Modern Square Dancers.
However, there's a lot of detail in that article and you might find it overwhelming — if so, please don't worry about it and just come join the dance. It is not necessary to prepare for the YRECD class.