How can I speak German very well?

The most important thing is to speak from the beginning, as simple as that sounds.

To quickly learn to speak German, you should read aloud, pronounce, and apply the words and sentences you learn from the very beginning. At first, there will be misunderstandings, but it's best to immediately try out everything you've learned in practice.
You can also mix with English or other languages that your conversation partner speaks. Over time, you increase the proportion of German words and phrases. You should not be afraid of making mistakes. When speaking, you don't have enough time to always think about correct grammar. So, you should communicate and accept mistakes for the time being. You can work on grammar separately in a German course.

Learning a language is similar to learning an instrument: While you practice a piece of music slowly (comparable to sentences and phrases) or train individual grammar elements (just like a scale when learning an instrument), you can't just stop while speaking, just as you continue playing a piece of music even if you make a mistake.

It certainly helps if you speak with other people who know German. You should also never hesitate to have a little chat with strangers - with the neighbor, the nice colleague, the bartender, or the friendly salesperson in the store.