There are several companies that operate ferry trips from Helsinki to Tallinn.
You can choose tickets: ordinary ticket for a person, one-day cruise (there and back in one day), tickets for people + car, if you want to travel by car in Estonia or go somewhere else. Tickets are bought online, if you take with a car, then of course you will need to enter the car data. If you decide to drive a different car, you must contact the company and change the old data to new ones! We drove the car for the first time, and there is nothing difficult there. The most important thing is that you should arrive at the terminal and stand in line no later than 1 hour! I recommend arriving in advance, at least 1.5 hours.
You have to stop at the window, the employees will ask for your car number, documents and tell you to stand in a line. An important note: there seems to be no toilet there, so think about it in advance. When they start to start the cars inside the ferry, just follow the cars in front and look for the people in the bright green jackets, they show you where to go.
You already park inside the ferry, and go out for a walk. It is not possible to stay in the car during the trip. You will not be able to return to the car either, the doors to the car compartment open 15 minutes before arrival. My advice, there is not much seating on the Viking ferry, many people were lying directly not on the floor. So the first thing you should do when you get on the ferry is to find a seat, it can be seats in the restaurant or just seats on the ferry (and there are not many of them!)
Of course on the ferry you can go to the top deck and there are also shops, one with perfumes, alcohol and sweets and another with clothes and accessories. Both stores are not big. Maximum for 30-40 minutes.