I don't have a solid answer for you here, because I don't think there IS a single universal answer. But I do have some advice.
Cutting right to the chase, I really, deeply believe that the divide between "secular" and "Christian" is false. I believe it is a division made by people and not by God. And the definition of a Christian author or band is vague anyway; just because something comes from a Christian record label or publisher does not mean it is high quality, doctrinally sound or of value, and just because it doesn't doesn't mean it's not. Make sense?
I became a Christian about when you did, and something I deeply wish I'd taken hold of early on is a healthy, accurate view of God. God is bigger than any human idea of big, and His work is not found only in churches and Christian bookstores. God's fingerprints are all over this world for those who look. There is more to hearing God's voice in the world than looking only to things that have a "Christian" stamp on them.
Am I saying "watch/read/listen to anything you want"? No. Give this some serious prayer and don't do anything that'd cause you to stumble into sin. But also consider what that means; no one is responsible for what you do and who you allow yourself to become but you. Hearing profanity will not automatically make you swear. Seeing violence will not automatically make you violent. Jesus said what goes in does not corrupt a person, but what comes out. But you SHOULD avoid anything that pushes your buttons and makes you WANT to sin. Sadly there are no hard and fast rules (or few), as this is different for everyone. It's a matter of seeking God and searching your own heart.
In any case, you WILL NOT go to Hell for watching the wrong movie, hearing the wrong song or reading the wrong book.
What I would suggest is this:
1) Study your scripture, and spend some time in prayer. Talk to God. Listen for Him. The primary task of a new believer is to get to know God and to flesh out what loving and following Him means. Rest assured, He WILL surprise you, whatever He does. I can also recommend some good books, since you, like me, seem to be a bookworm.
2) Consider the things you're struggling with (Harry Potter, etc.). If reading/listening/etc. worries you, consider why. Is it because your conscience is being pricked, and God is nudging you away from the thing in question, or because you're afraid of what some other Christian person might think? If it's the first one, put whatever it is down, period.
3) Try to learn to really SEE what you're reading/watching/listening to. Look at it. Like I said, God's fingerprints are everywhere. When sin, or virtue is portrayed, look at HOW. A thing can, for example, be violent and be speaking AGAINST violence at the same time. Try to figure out what's being said by looking deeper. Look for where Christian ideas, like love for others, sacrifice for others' sake, forgiveness and love for enemies, redemption, and acceptance of those the World hates, appear. Look at how they're shown. The places I see God show up never ceases to surprise me.
4) Cultivate an accurate idea of what "Worldly" means. The idea that Worldy means "enjoying anything non Christians like or produce" is simply not scriptural, or (IMO) healthy. The Pharasees set themselves apart from the "world" more than anyone in Jesus' day, and they were arguably the most Worldly people He ran into. A Christian kid who only listens to hymns but looks down their nose at and shunns the kid in the Slipknot shirt is, in a way, being Worldly. Because Worldliness is all about your attitude toward others, and God.
5) Realize this is a process. You are NOT going to be perfect overnight. Or Ever. And God doesn't expect you to be. If people could be perfect of their own power, Jesus came and died for nothing (as Galatians says). You are in a growing process. It is a process that will take the rest of your life. Remember that any righteousness we have comes from Christ, and that He's not the kind of tempermental god who's sitting in Heaven waiting for you to mess up. He's nuts about you. He wants what's best. And he's not going to give up on you. He's your Father.... not your Employer.
Okay, this has run REALLY long so I'll end by saying this; a LOT of people settle for a shallow faith built on what certain Christians say they're supposed to do. Your fellow believers can be and often WILL be a source of great encouragment, strength, wisdom and grace. Just beware those who have an unhealthy fondness for rulemongering.
If I've been in any way unclear, you have any questions or just want to discuss this more one on one, by all means, PM me, please. I may've just talked your virtual ear off but I promise I'm also fully capable of listening