Use functions from the sp and rgdal packages. Assuming your shape file is named myshp
library(sp)
library(rgdal)
proj4string(myshp) # Gives the CRS of the data
myshp=spTransform(myshp,CRS("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")) # To convert it to WGS84
ggplot2 input has to be a DataFrame, to convert a SpatialDataFrame to a DataFrame named mydf, run
library(ggplot2)
mydf=fortify(myshp,byid="your id variable") # Converting the SpatialDataFrame to a DataFrame
myshp$id=row.names(myshp) # Adding your SpatialDataFrame data to the DataFrame
mydf=merge(mydf,myshp@data,by="id",all=T)