The most important thing I can tell you as you head to college is that God is already there! He hasn't abandoned you; He loves you and wants you to walk with Him every day. You can trust God's promise: "Never will I leave you; never will I forsake you" (Hebrews 13:5).
What can you do -- right now -- to prepare for the challenges you'll face? First, make sure of your commitment to Jesus Christ. Is He real to you, and are you seeking to follow Him as your Savior and Lord? If you have never done so, now is the time to invite Him into your heart and commit your life into His hands.
Then ask God to lead you to others on your campus who love Christ and are seeking to follow Him. Most campuses have active Christian groups; seek one where you can have fellowship with students who are facing the same problems you'll be facing. The Bible says, "Therefore encourage one another and build each other up" (1 Thessalonians 5:11).
Finally, seek God's will for your life. Ask Him to guide you every day, and to help you resist the temptations and challenges you'll probably face. Ask Him also to guide you as you face major decisions: who you'll marry, what career you'll pursue, etc. God's promise is for you, and for all who truly seek His will: "I guide you in the way of wisdom and lead you along straight paths" (Proverbs 4:11).