Apps might be helpful but aren’t really necessary. It’s easy, and if you get confused just ask. They all speak English. Some better than others but you shouldn’t have any problems.

You can buy your tickets here: https://www.bahn.com/en/view/index.shtml
or directly at the station. Prices will be about the same.

Remember that you’re going from Frankfurt on the ‘Main’ river. (NOT Frankfurt am ‘Oder’ - which is in eastern part of Germany, former DDR)
HBF or Hauptbahnhof means Main Train Station.
Not sure which hotel y’all stay at but I typically go to the main station which is near Köln’s Cathedral and then I walk to our hotel (Maritim). Hope this helps.