Valid client and signature using ggmap package in R - r

Question

I am using ggmap package in R software in order to geocode a list of addresses. The counting is over 2500 - Googles diary quota for free users.
I have been searched about it and I found out that it is possible to geocode 100 thousand addresses per day if you have an account. I decided to open an account and I received a message that I have 60 trial days and they gave a 300 dollars credit. Despite this, I didnt discover where I can find the "client" and "signature" options that ggmap requires.
If someone can tell me where I can find it, please let me know!
My aim is to make Google searches by the address. If address is not recognized, then I make it searches for the local using the zip code. Here is what I am doing:
library(RJSONIO)
library(RCurl)
getGeoData <- function(location, api_key){
location <- gsub(' ','+',location)
geo_data <- getURL(paste("https://maps.googleapis.com/maps/api/geocode/json?address=",location,sprintf("&key=%s",api_key), sep=""))
geo_data <- fromJSON(geo_data)
if(geo_data$status=="ZERO_RESULTS"){
return(NA)
} else return(geo_data$results[[1]]$geometry$location)
}
New error:
getGeoData(basefinal$cep[6793], "MY KEY")[[2]]
Error in getGeoData(basefinal$cep[6793], "MY KEY")[[2]] :
subscript out of bounds