How do you transfer my songs from PC to iPhone 3gs?

Plug your iphone in using USB cable to the computer and it should automatically sync. You can click on your iphone in the left set of links, it's probably a personal name. Then use the tabs to find photos or sync and sync them. But be careful not to back up on a computer that is not your own, you will not be able to use yours.

Check the "import" function. You import files/songs into iTunes.

And don't auto sync, otherwise, iTunes will replace all iphone existing songs.

I've suffered loss of all my iphone music files. It's so upsetting.

There is a step by step instruction on how to transfer songs/music from iphone to iTunes on PC.