From Ohr Avigdor Shaar Habitachon by Rabbi Avigdor Miller
When you settle down to the actual subject of serving Hashem, the very first of all things that you need, is the subject of bitachon (trusting Hashem). That may seem at first like a new idea, because after we make up our minds to serve Hashem, we instinctively look for action — for things to do, obligations that we have to fulfill.
Instead the author of Chovos Halevavos gives us things to think about. So we begin to see that the most important part of serving Hashem is thinking. We also have functions to perform, but first on the list of avodas (service of) Hashem is certain attitudes of the mind that we have to acquire.
The most important part of serving Hashem is thinking
Above all, the most important requirement of avodas Elokim is bitachon, which is a matter of thought, of the mind. That’s the first and most important way to serve Hashem. You can also refer to this idea with the word emunah (sincere belief). This means if you want to serve Hashem, work on emunah. That’s number one.
Admittedly, emunah is a prerequisite for avodas Elokim. But suppose a person reads the whole Shaar on avodas Elokim and is inspired to do something now. The author tells him, if you want something to do, gain more emunah. You will discover that the emunah you had before is too weak, too small to be called part of a real service of Hashem.
Now bitachon is just one form of emunah. Bitachon tells us to know that Hashem is in charge. It also tells us that Hashem wants to do kindness with His Creation, He is a chafetz chesed (desires kindness). Once you have these two ideas established in your mind, you have achieved the emunah that you need to serve Hashem. So we have to teach ourselves (1) that Hashem does everything; whatever happens comes from Him. And we have to learn (2) that Hashem is doing it all for the purpose of chesed, kindness.