To travel from London to Paris by train, you have to use the direct Eurostar train. The journey time is about 2:30 h.
Buy your train ticket for Eurostar and find schedules via the given booking links. As earlier you book, as cheaper the ticket prices are.
An other travel option is to use a bus. The journey is much longer, but usually cheaper. Flixbus runs this route several times per day, but as well other bus companies.
The journey by train and including the ferry Dover (United Kingdom) to Calais (France) is for now no nice option anymore. The journey is long and the ferry got really expensive for foot passengers since several years.
To buy train tickets from France to Germany, compare the booking links we list. Prices are usually the same, but sometimes vary slightly.
Depending on where you start in France (and where you want to travel to in Germany), you can either use the route via Paris by Thalys to Cologne. Or from Paris via Strasbourg/Saarbrücken to Frankfurt or Stuttgart by TGV/ICE. There is also a direct train from Marseille in direction of Frankfurt.
If you want to use a night train, have a look at the Russian night train Paris to Moscow (exit in Berlin). A very nice experience and high quality.
The Europe Special price of Deutsche Bahn is available from EUR 39. As earlier you book, as cheaper the prices are.