1) God's glory refers both to His intrinsic attributes as well as the glory ascribed to Him by His creatures. We give God glory by recognizing and praising His attributes such as holiness, mercy, justice, faithfulness and more. 
2) The primary purpose for humanity is to glorify God in all that we do. We glorify Him by magnifying His greatness before the world through our words and actions.  
3) We should give God glory because He created us and everything else for His purposes. Since He made us and gives us all things, we ought to praise and honor Him with our lives.