I have faith in Allah and His existence, but I have a question:
Allah created angels and humans to worship him. Allah is self-sufficient and doesn't need anything, then why the command to Creation to worship Him?
To answer this question there are things you need to know like the following versus:
2:29 It is He who created for you all of that which is on the earth. Then He directed >Himself to the heaven, [His being above all creation], and made them seven heavens, and He >is Knowing of all things.2:30 And [mention, O Muhammad], when your Lord said to the >angels, "Indeed, I will make upon the earth a successive authority." They said, "Will You >place upon it one who causes corruption therein and sheds blood, while we declare Your >praise and sanctify You?" Allah said, "Indeed, I know that which you do not know."
Therefore we do not know why we have been created neither angels do. There is no one worthy of worship but god, this life is all one big test for us and it is very short considering after death, how would you ultimately know what is good and what gains you sins if you have not followed the guidelines of a religion, therefore the holy books that where sent down to us are our guidance and yet it is another test to mankind as you see there are believers and unbelievers (winners/losers).
Angels main duties are to worship god they do not have any desires, different angels can have different duties like the angel of death and those who record our actions.
Allah has commanded us to worship him he is the all knowing therefore we worship him, god has sent us signs and knowledge which strengthens our believe, which is more than enough for us to believe, part of believing is following what god has commanded, humans to worship God, us as humans we know so little our simple minds may not understand certain things, our duty for now is to worship god, some things we don't know the answer for, please refer to the versus below
He has commanded that you only worship Him; that is the right religion, but most men do >not understand ". (Surah Yusuf 12:40)
As a very short answer, I can say that: perhaps, a reason of the mentioned issue is that: He wants us to worship him as an exam. Actually he intends to test us by this way and see that which people are appreciative and worship him as an exam, otherwise as you mentioned, Allah is self-sufficient and doesn't need anything ...
You can see the following sites which could explain more regarding the mentioned matter.