Hi Nicolai,
Welcome to the RStudio Community!
As a general rule, you will probably get your data in an excel file or a csv file, and the readr package is an excellent tool for data importing.
Once you have the data in R, you will want to check for missing data with the naniar package, and explore your data with the DataExplorer and skimr packages.
You may find that you need to make your data more tidy (https://vita.had.co.nz/papers/tidy-data.pdf ), in which case you may need to use the tidyr and dplyr packages to rearrange your data.
For forecasting, there are several choices, but if it is a time series, the tsibble and fable packages are popular.
By the way, specific questions about homework can not be answered here, but general inquiries (like this) are welcome.
Your professor would likely also appreciate it if you asked some of these questions at office hours. Give it a try.