How to download music from your computer to your iphone?

Is there a way how I can 'convert' music from itunes on my computer to itunes on my iphone 4s?
Or is there a way to download music from youtube to my itunes library on my iphone?
Downloading music from itunes is just getting way too expensive.

1. Open File, choose Add File to Library then choose the folder where your music is saved, select the songs that you want then click Open to copy the songs to the iTunes library.

2. Click on your iPhone name, choose Music tab, click on Sync Music, Selected Music then select the playlist you want to sync to your iPhone then click Apply.

3. Click on your iPhones name, click Photos tab.

4. Choose Sync Photo From, click Choose Folder and select your images location. Then click Sync.

5. Open My Computer, go to your movies folder and choose the movies you want to copy to your iPhone. Right click then choose Copy.

6. Click on Movies in iTunes Library, right click and Paste the movies.

7. Click on your iPhone name in iTunes, choose Video tab and click on Sync Movies, choose the movies you want to sync to your iPhone. Then click Sync.

Go through the user manual:

http://manuals.info.apple.com/..._guide.pdf

You may try this YouTube downloader, I know it can help you download youtube video to computer for free. You can also use it to extract the audio from YouTube video as MP3 songs, and then you can import the mp3 songs into iTunes and sync them to your iPhone.

Try chrome extension Youtube Center. It can do much more than just download videos.

I often use Any Video Converter Free to download youtube music to computer and then add then to iTunes and then sync my iPhone with it to get the music on my iPhone. Have a try! It works pretty well.