This question has many ways to answer, but here is a very, very basic answer.

This is one of the examples provided in the help files for ARIMA in the forecast package:

fit <- arima(WWWusage,order=c(3,1,0))

Suppose you want the original series in red and the fitted series in blue:

plot(fit$x,col="red")
lines(fitted(fit),col="blue")

You can try out this commands and check the output!