ITunes 11 won't let me sync downloaded music to my iPhone?

I downloaded iTunes 11 yesterday and any new music I add to iTunes won't sync to my iPhone.
The music in question are songs that I did not buy from iTunes; I either got it from a friend or downloaded it myself.
What do I do?

Added (1). Another important note:
The songs in question SHOW UP on my phone after I sync it; I get a message saying that there were *insert number here* problems, blah blah blah, etc. And when I go to one of the (for lack of a better word) "problem" songs, they are unable to be pressed/played.

Sincerely, I don't know. I installed iTunes 11 and wasted 5 minutes trying to find out how to manage my iPhone (should have watched the tutorial, but don't have time). I had the impression Apple wants you to exclusively focus on their store. I had the feeling I was trapped in their shop.
So, I uninstalled iTunes 11 and reinstalled the version 10 (10.7.0.21). After starting the old version, got a message saying that the iTunes Library could not be opened as it was created by a newer version and iTunes won't start.
I went to the location of the iTunes Library (C:\Users\your_username\Music\iTunes) copied the "Album Artwork", "iTunes Media" and "Mobile Applications" into a temporary folder (it may be a big amount of data), then deleted the iTunes folder, started iTunes (no problem this time) and copied back the three folder above.
Until someone more skilled than me answers, you can try this method.
I updated to iOS6 and got really disappointed (I used Google Maps a lot); no way to return to iOS5! Now got disappointed with iTunes 11. I will never, ever install a new version of whatsoever program of Apple without really reading all the reviews.

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