The default folder for iTunes is My Music\iTunes\iTunes Media
If your music is elsewhere you can tell iTunes where it is.
1. Open iTunes.
2. Edit->Preferences, Advanced tab.
3. Uncheck Keep iTunes media folder organized if you are picky about song locations and have your own organization scheme. If you check it iTunes will put the songs in folders Artist\Album\track# Track Name.ext
4. Uncheck Copy files to iTunes Media folder when adding to library if you unchecked step 3. Check this if you checked step 3. Unchecking this means you will have to put your music where you want it in your organization scheme before importing it into iTunes.
5. Click on the change button at the top, navigate to the top of your music folders. Select the folder above your music. So if your music is in E:\Datafiles\Music\Rock and you have Classical and Country under the Music folder, you would choose Music as your location.
6. Click OK