Are you using RStudio? I would suggest installing RStudio and creating an RStudio Project. Then you can open an R script, type your code there and save it as a file.
To use a custom title for a plot: plot(generationData$Price, xaxt = 'no', xlim = c(1,50), main="My Title"). You could also do:
plot(generationData$Price, xaxt="n", main=NA)
title("My Title")
The acf plot uses R's "base" graphics system. ggtitle is from the ggplot2 package, which is a completely separate plotting system that uses R's "grid" graphics system and also has its own functions and grammar.
It might be helpful to check out the free book R for Data Science to learn more about how to analyze and visualize data with R (as well as the workflow of setting up R scripts and projects), and the free book Forecasting: Principals and Practice, 3rd ed. to learn more about forecasting with R.