If you believe in the existence of god then you are operating on faith and no reason is necessary nor is any reasonable explanation for the world as we know it possible.
If you seek an explanation then your best bet is to investigate what science has thus far been able to reveal and relate to human understanding.
If you are undecided as to whether or not god exists, start by seeking a definition for what the term "god" refers to and check to see if it agrees with and corresponds to the knowledge you have thus far gained. This in turn requires that you first develop the ability to differentiate between what you believe and what you actually know. Unfortunately this is not a skill that is promoted by those who would rather have you believe what they say regardless of its validity, therefore, caution is advised.