Although the Bible doesn't go into detail about the devil's origin, it does indicate that he was an angel who rebelled against God and was cast out of heaven as a result: "How art thou fallen from heaven, O Lucifer, son of the morning!" (Isaiah 14:12, KJV).
Why did he rebel against God? The reason can be summarized in one word: pride. He became jealous of God's position, and in his pride he decided to lead a revolution and take God's place: "You said in your heart, 'I will ascend to heaven. ... I will make myself like the Most High'" (Isaiah 14:13-14). He failed - but his goal is still the same: to take God's place.
You need to remember two important truths about Satan. First, he is real, and he is powerful. Don't make the mistake of thinking he is a cute little cartoon figure with a red suit and a pitchfork - for he isn't. He is completely evil, and his goal is to block God's work in our lives. The Bible says he is "like a roaring lion looking for someone to devour" (1 Peter 5:8).
But the second thing you need to remember is that the devil is a defeated foe! By His death and resurrection Jesus Christ conquered sin and death and Hell - and someday Christ will come again to pronounce judgment on Satan and his followers. On whose side are you - Christ's or Satan's? Don't give in to the devil's lies, but commit your life to Christ and become His follower.