1.) Abraham believed and trusted God, even in the face of what seemed impossible, and his faith was credited to him as righteousness. Against all hope, Abraham believed and became the father of many nations (4:18). Abraham was justified in God's sight because he trusted God and believed that God could/would do what He promised.
2.) We, too, are justified by faith. Jesus came to give Himself as a sacrifice of atonement, and in this we have faith - that His death was sufficient payment for our sins, and that His resurrection from the dead gives us eternal life because He conquered the grave so that we also will live, as He lives. We have faith in God's promises in Jesus, that if we believe in Him, we will be raised to eternal life with Him.
3.) When we believe in Jesus, we are acquitted of the crime of sin. For our sin, we're deserving of punishment and God's wrath. But because Jesus took the punishment for us, and paid the penalty for our crimes on our behalf, we've been made righteous in the eyes of God. We are justified and restored to a right standing with Him. I dare say this justification gives us a clean slate, as if we'd never sinned.