How to start writing a book?

Writing is a craft you have to learn. So no, it’s not easy to write a good book, but if you really want it, you can do it. However, as so often in life, there is not the right method to write a novel. You’ll have to find your own.
But here are some tips on your questions:

But I wonder how to start it best?

For the first time, you have made important reflections: as I understand you, you have a basic idea of what you want to write. As it goes on from here, something depends on the type:

There are authors who just write down. That means they’d just sit on the computer and look where they’re coming in the end. Some of them write chronologically, others write the scene they just like, and then gradually connect everything to a continuous story.

Other authors would further develop the basic idea before writing. That means they would now consider their protagonist’s way: where does he start? Where does he end? What obstacles does he overcome? Planning can be carried out so far that you have to write down everything in the end. There are also thousands of different tools, such as figure databases, mind maps or a time stream.

What kind of type I am, I found out that I just tried it out: I just wrote on it, produced absolute junk and so slowly figured out what I need to plan to write better works.
You’re not going to get around either. Try out and don’t be afraid to start again – if you find it necessary.

I have the feeling it might end too soon

What do you mean “end too soon”? That your story hasn’t been counted yet? Or do you worry that the scale is not enough in the end?

I’d advise you, don’t worry about the length of your story. She’ll be as long as she’ll be long. You don’t know what’s going on for your basic idea. If it’s a short story, it’s all right.

Don’t try to force you to come out on devils, write something that doesn’t give you this idea. Tell the story you think it’s right. There is nothing at the end of thirty pages to fill with any garbage, just to get to the minimum number of words or pages.

In principle, however, you can first say that the length of your story is determined by two things: content and writing style.
In addition to the main act, novels usually have minor acts or several obstacles on the path of the protagonist. Of course, Dadruch also automatically has more content to write. For example, in Harry Potter you have side-shops (e.g.: school, love) and many obstacles (e.g. seven horcrxes and not just one). The same thing about Tribute of Panem: One has side acts (e.g. Gale or Peter) and many obstacles (e.g. the twenty-four tributes and the way to kill them). The latter is particularly clear when one compares tributes of Panem 1 and X.
In short stories there are usually no side conflicts. There’s no room for them. The same applies to obstacles. There is less and they are partially overcome faster.
Otherwise, of course, your style of writing makes a lot of length: some describe quite a lot, others describe rather little. Again, there is not the perfect measure. It’s just a taste thing, but there’s also a thick bandwidth, which is good for most people. At the beginning, however, one tends to describe too little.

maintain tension

As you get tension in history, of course, depends very much on yours. It helps to take the readers into history. A good style of writing helps.

So try to describe everything as visually as possible for your readers: the environment, the situations, the figures. Always stick to the principle “Show, don’t tell”. That’s supposed to be so much, show me how something is and don’t tell me. Example is written: The main character has strict parents. This does not mean that the readers perceive them. In the end, the actions of parents make these strict or even loose. No matter what was written for a sentence at the beginning.

But your writing style will not be perfect from today to tomorrow. Here too: writing, falling, learning and doing better.
When writing, best reflect on yourself. Read your work later and consider if you would buy it in the bookstore. Write out points you want to improve and work them gradually. Just for as general things as “describing better places or figures” you should also take time. You probably won’t learn that in three days.
You don’t know what to improve, get someone else’s feedback. You yourself like to become a little operationally blind. Show your story to any friends or look for people who give you feedback. For example, I have already written something about some Wattpad stories here. In some cases, however, the questions are partially deleted as advertising.

And still as a small tip without wanting to kick your feet: You have built a bandworm set up in your text. These can be quite exhausting when reading. This applies especially if no commas are set there – as with you.
Make sure you write your story. The best novel is not read when you are constantly disturbed by errors in the reading flow.