Considering that we are all made up of the same 4 nucleotides in our DNA, the same 4 nucleotides in our RNA, and the same 20 amino acids in our proteins, why are we so different from each other?

The order that the nucleotides are arranged in form different "codes" to create traits. RNA in humans is used for transporting, and making the amino acid proteins. The order of amino acid proteins also changes the polypeptide formed, and changes more traits, and characteristics. Basically, it's like reading a book. Each book is made up of the same 26 letters, but their all arranged in different orders. You can find the some of the same words in another book, but no two books are exactly alike. All the letters and words are arranged in different ways, even though other books have the same letters and words.