Done a few crossing round that area. As been said, just turn up, on a bike. Pay the money, and they will get you on. One crossing I was on, there must have been about 50 bike's on board. All lashed up to the side. As all the ring's where taken. Never yet found a ferry crossing that they could not get an extra bike on. Don't worry, you will be all right.
To buy petrol in Europe. Pull up at station. Wait. Get out a 20 Euro note, then ask someone to fill up the bike. Give person money. Ride away. Simple.