Upvote:5
All the seats coloured in dark grey are still available. Clicking on one of them will turn the respective seat green, indicating that you have successfully chosen the seat. Then choose "Auswahl ΓΌbernehmen" in the lower right corner to confirm your choice and continue with the booking process.
As far as I can tell from the seat map, the seats on the left side (in the area with arrows pointing up) are on the upper level. The front of the bus is to the right which means you should choose seat 41 or 45, located in row one on the upper level. However, seats in the first row usually have a barrier in front of your legs, limiting your legroom. Sitting in any other seat, you would be able to slide your feet underneath the seat in front of you.
On a side note, I would recommend booking on Deutsche Bahn's English website in order to see all relevant instructions in English.