Music not going onto my iPhone?

So I had my brother put about 2500 songs on a flash drive for me so I can put them onto my iphone 4s through iTunes.
All the songs are currently on iTunes, but they won't sync to my phone. I don't know why. I've tried everything I could think of.
Can someone help me figure it out?

Maybe there's too many songs? I really don't know sorry. But i suggest you just buy a itunes card and use it sometimes downloading music can be pretty annoying and get you into trouble

Those songs must be moved from the flash drive to the iTunes library on the PC. If they're not, iTunes can't sync. You must "Import" the songs to the iTunes library before you can sync the phone.

Open Explorer and find the folder My Documents\My Music\iTunes\iTunes Music

Drag the songs from the flash drive to that folder. Open iTunes. ITunes will now find the songs. Now sync the phone.