"'Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.'" NIV translation
Once again, we see that sin has a price and so Cain must wander the earth. Cain had trusted in the work of his hand in bringing crops as an offering to God and now he will not be able to grow crops. God is giving him the chance to see and understand that it is He that is the provider of all things. This is the same thing that all of us (even Christians) struggle to understand.
"Cain said to the Lord, 'My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.'" NIV translation
Cain pleads for God's mercy but he does so without repentance. In his plea, Cain never confesses his sin and so there cannot be forgiveness. The same thing is true for us, in that, we cannot see our need for a savior without a change of our minds (repentance).
"But the Lord said to him, 'Not so, if anyone kills Cain, he will suffer vengeance seven times over.' Then the Lord put a mark on Cain so that no one who found him would kill him." NIV translation
Cain feared for his life but God reassured him that his life would be spared even though he deserved death. Anyone who killed him would be completely dealt with by God as He put a supernatural mark on Cain to ensure his life. This is a picture of what was to come in the Passover (see Exodus 12) and is to come when God puts his seal on the witnesses in the last days (see Revelation 7).
"So Cain went out from the Lord's presence and lived in the land of Nod, east of Eden." NIV translation
And so Cain went to live in the land of Nod (wandering).
"Cain lay with his wife, and she became pregnant and gave birth to Enoch. Cain was then building a city, and he named it after his son Enoch." NIV translation
Once again we see the disobedience of Cain in that he was building a city even though God had said that he would be a wanderer. He had a son and named his work (the city) after Enoch.