Although Satan is a spiritual instead of a physical being, the Bible does tell us that at times he can assume physical form. For example, when Satan tempted Jesus to turn away from God, he may have taken on the appearance of a man -- although with supernatural powers (see Matthew 4:1-11).
But the Bible doesn't tell us exactly what Satan looks like when he does put in an appearance. The Apostle Paul warned that at times Satan may seem like an angel of light -- in other words, appearing to be pure and good in order to deceive us into thinking his ways are best (see 2 Corinthians 11:14). At other times, however, his appearance must be frightening and evil -- because that is Satan's true nature, whether he is seen or unseen. The Bible warns, "Your enemy the devil prowls around like a roaring lion looking for someone to devour" (1 Peter 5:8).
Always, however, remember two great truths about Satan. First, he is absolutely opposed to God, and to God's plan for our lives. And because of that, he'll do everything he possibly can to keep us under his control and away from God. Don't be deceived, however; his plans for us are always evil.
Second, never forget that although Satan is real, he is a defeated foe -- because by His death and resurrection Jesus Christ has defeated death and Hell and Satan. Put your faith and trust in Christ, for some day the final battle will be fought and Satan's power will be destroyed forever.