As you are knew to R, I would recommend a few resource on model with R:
Chapter on model in R4DS book
Chapter on model in Modern Dive book
https://moderndive.com/6-regression.html
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!