I need to import two columns (time and temp) worth of data from a .csv file into R and convert it to a time-series and the plot it with a linear regression line. So far this is my approach:

You'll find in both book how to prepare data and how to model with R.

Regarding your issue here, it is kind of difficult to help you because we do not know what Wintemp is. It would help if you could adapt your example to provide a reprex. Thank you!