1 Peter 5:8 (ESV)
Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour.
This verse can scare the bejeebish out of you. Depending on where you are spiritually, and what your background might be, your vision of Satan, his minions, and how active a role (if any) they play will differ a lot.
Have you ever seen a lion on the prowl? He paces back and forth, he stoops and watches, and he is ready to pounce and kill his prey at a second's notice. This is the image that Paul is giving Peter here. Satan is actively waiting for you to allow him to come in and pounce on you, to destroy your faith, and to kill your relationship with the Father. That is why Paul tells Peter to be diligent, and to be aware of what's going on. This means through prayer, through study, and most of all, through Faith.
Satan is real, and he hates you, he hates that you love the Lord, and he even hates those of you who don't know the Lord yet, and are thinking about making a decision for Him. He wants you to worship him, and to give your life over to the worldly things he controls. Don't give in, be self-controlled and alert, and keep your eyes focused on Jesus. Remember, also, that we have the power to put Satan down through the name of Jesus, and you can practice that in prayer.
Dear Lord, bind Satan in our live, let him have no power to destroy or harm us. Give us the power to be self-controlled and alert, and diligent in our walks with you. In the name of Jesus, Amen.