My iPhone 4 won't let me put music on it from my computer?

So i unlocked my iPhone today & now i want to download music on it when i drag it to my phone it won't let me put it in my iPhone. What do i do?

I'm going to assume you have never put music on an iPhone/iPod or used certain features in iTunes so pardon me if you have-there isn't enough information in your question for me. You have to have music in your iTunes account. You can put music into your iTunes account by buying it from iTunes, ripping it from a CD into your account, or transferring it from somewhere else on your computer. Once it is in your iTunes account, it is merely in your library. You have to create playlists and then sync your device-that's how it gets onto your iPhone/iPod. Now, there are ways to drag and drop music onto your device, I just never used this method I usually sync and make sure my playlists are squared away. That's why I say I'm assuming things about you. When you drag and drop it has to be a certain way and from within iTunes and your iTunes library, you can't drag a song from windows media player, for example, and drag it over to your iTunes account where you have your iphone plugged in and syncing and drop it on to the iPhone device that's on the left hand side.
If this doesn't help, please give additional details about exactly what you already tried.