J. C. Ryle answers the question in several ways in his treatise on the subject.
“First, holiness is the habit of being of one mind with God, according as we find His mind described in Scripture. It is the habit of agreeing in God’s judgment–hating what He hates, loving what He loves–and measuring everything in this world by the standard of His Word. He who most entirely agrees with God, he is the most holy man.”
Are you seeking the mind of God in His word and pleading with Him for a spirit of agreement in your entire being? This is part of what it means to be holy.