What do I think about when I meditate?
It's important to remember that you aren't trying not to think. This will only make you think more. What you are trying to do is to calm the mind, and to eventually still the mind. I like to think of my mind in meditation like the surface of a still pond. A slight breeze or a falling leaf will create movement, a rippling outward. But the ripples will expend their energy if let go, and if they don't run into anything along the way, and it will become still once more. A thought falling into the surface of your mind will do the same. Allow your thoughts to go when they come in like this, rather than reprimanding yourself for thinking. And bring the focus back to the calm stillness of the surface of the mind.
I can't clear my mind! Meditation isn't for me!
That isn't a question. But I have an answer for you anyway. Just like I said above, it isn't your job to clear your mind. This is a misconception and a deterrent to beginning your practice. It's only your job to slow the mind. And you can slow the mind by finding an anchor for your meditation: a word you repeat in your mind, the breath, a sensation in your body. Also, you can hold this affirmation in your mind or practice writing it: Meditation is for me. I can still my mind.
Is silence or music better?
I believe some silence is really necessary to cultivate a great relationship with yourself. If you can't stand silence, there's probably a problem somewhere. And that's ok! But that's what you want to address, and beginning with a little silence, focused and fully present, can be a great start for you. That being said, I often use music or guided meditations in the following situations:
Should I be seeing things or having visions?
Maybe! I like to work on "clearing" my mind, meaning that the excessive spinning that the mind is usually in has slowed down to what feels like a very calm turning to me, and there are moments I think of as "stillpoints". Inside of these stillpoints, I have a feeling of deep connection and understanding with The Universe itself, or Spirit. I can also have a sudden thought or vision drop in at these moments, and I know that these are messages or guidance from my Higher Self or The Universe. They aren't driven by ego, because they came at a time when the ego wasn't running the show, and I was most connected to my Higher Self. Some people experience intense visions in meditation, and it's very common to begin by seeing colors- purples and blues will usually come first.
Should my back hurt like this?
Erm, maybe. There is this thing called "meditator's back" which can happen in the first 3-6 months generally of establishing a true practice, especially if you're practicing between 30 minutes and 3 hours a day. No, I've never practiced for 3 hours in one day! But when I started, I had a moving practice (qi gong and tai chi) because I had a serious back and neck problem and couldn't stand the stillness. Eventually, I incorporated a short 10-minute sitting practice at the end of a 45-minute movement practice. Then I worked up to 20, and now I can sit still for 45 minutes just flying high in the ethers. This practice is FOR YOU. If you have a real back or neck problem, support yourself! There's a form and style of meditation that WILL work for you. You just might have to find that right combination of bolsters, chairs, blankets, cushions, or movements. But also, if it just feels like basic muscular pain kinda like when you work out...just deal with it and it'll eventually go away. You're building up muscle fiber that will literally support your practice (and life) going forward. :)