Transferring your iTunes to a new computer does not have to be complicated. However, trying to do so by simply exporting your iTunes library will not work, because the library's database file does not contain your media content; it only points to it.

Organize

Launch iTunes and click on "Preferences" from the iTunes "Edit" drop-down menu (for Windows). For Mac users, click on the "iTunes" menu and select "Preferences." You will see the "Storage Settings" for your media. Enable the "Keep iTunes Media folder organized" option. Enable the "Copy files to iTunes Media folder when adding to library." This will make a copy of any media file you add to your library and store that copy in your iTunes Media folder. Click on the "File" menu, select the "Library" option and choose "Organize Library..." Click on the "Upgrade to iTunes Media Organization" button and confirm by pressing "OK." If your media is already organized, this option will be grayed out.

Consolidate

Connect an external hard drive to your old computer. You will use this to transfer all your content. Create a new folder in the external had drive where you will temporarily store your iTunes. Go to "Preferences" in the iTunes menu. Click on the "Advanced" menu. Under the "iTunes Media folder location" section, click on the "Change..." button. Select the new folder you created in your external hard drive. Confirm by clicking "OK" at the bottom of the screen. Click on the "File" menu, select the "Library" option again, then choose "Organize Library..." This time click on the "Consolidate Files" option. This grabs all the content in your iTunes library and puts it your iTunes Media folder, providing the necessary paths your iTunes library needs to work the way you are used to. Confirm by pressing "OK" and then "Consolidate." Depending on the amount of content, this process can range from minutes to hours.

Complete the Transfer

Close the iTunes application. Locate the iTunes folder in your old computer. For Windows, it is generally in the "My Music" folder. For Macs it is generally in the "Music" folder. Your "iTunes" folder should include an "iTunes Library" file, an "Album Artwork" folder, possibly a "Previous iTunes Libraries" folder and the "iTunes Music" or "iTunes Media.'" Copy all the files and folders in the "iTunes" folder, except the "iTunes Music" and/or "iTunes Media" folder. (Depending on your iTunes version, this last folder will be labeled "iTunes Media" or "iTunes Music.") Find your "Music" folder (for Macs) or "My Music" folder (for Windows) on your new computer. Paste the contents you just copied into the "iTunes" folder of your new computer. It will not overwrite any possible contents you may have on your new computer. It will only add to that. Click and drag the "iTunes Media" folder that was temporarily stored on your external hard drive into the new "iTunes" folder of your new computer. Launch iTunes on your new computer. Click on the "Advanced" tab in the "Preferences" window. Click on the "Change" button that corresponds to the "iTunes Media folder location" section. Browse through your files, and select your "iTunes" folder in your "My Music" folder (for Windows) or "Music" folder (for Macs). Confirm by pressing "OK."

Tips

To save space on you computer's hard drive, you could keep the iTunes Media folder, which contains all your media content, on your external hard drive.

Source:

iLounge: Transferring your iTunes Library

Apple Support: What are the iTunes library files?