THE EASY WAY... to transfer music from your pc to your iPhone
What you will be doing is transferring your music from your computer (pc) to iTunes and then from iTunes to your iPhone.
1) Open iTunes, click on 'File' at the top of the window and from the drop down pane select 'Add File to Library' if you want to add just one song or 'Add Folder to Library' to add a number of songs contained in a folder.
2) From the 'Add to Library' window (that just popped up) select the file or folder that you wish to place in the iTunes music library by highlighting it and clicking on the 'Select Folder' button at the bottom of the window. (For clarification: Your song(s) will now be in iTunes in the 'Music' folder under the 'Library' heading. If your iPhone is already plugged into your computer and you click on 'Music' under the 'Library' heading in iTunes a window may pop up suggesting something to the effect of, things have changed and would you like to sync - select ok and the job is done.)
3) Plug your iPhone into your computer, if you haven't done so already, highlight your iPhone under the 'Device' heading, then select 'Music' from the top bar.
4) You can now choose to transfer (Sync) your 'Entire music library' (best choice) or 'Selected playlists, artists, albums, and...' to your iPhone by checking off your choice and then clicking on the 'Apply' button at the bottom. NOTE: If you chose the 'Selected playlists, artists, albums, and...' then THAT IS ALL you will find on your iPhone.