how can I have my music imported into Itunes without using all my storage space on my computer

Helo Nati,
I understand that you out of space on your computer and can't download any new songs. Let's go over some information about your iTunes library.
The first part about your products serial number is in the link below. Just select the type of device you have and it will show you where that is.
Find the serial number of your Apple product
https://support.apple.com/en-us/HT204308​
Now on to the iTunes question of where information is stored. That all depends on where you have selected the iTunes Media Folder is located. Take a look at the article below that goes into organizing your library as well as keeping it organized. It will show you how to make sure that the media files stay on your external when importing new tracks.
Locate and organize your iTunes media files
https://support.apple.com/en-us/HT204668
Now one other thing is if you have an iOS device, your backups are stored on your main drive. If you have relatively large backups, that could be the reason why the storage is full. Take a look at the next article that details where the location of your backups are. If you need to, you can delete them right in iTunes.
Find and manage your iTunes backups
https://support.apple.com/en-us/HT204215
Let me know if this helps.
Take care

You can certainly configure iTunes so that it does not make use of your system (C:) drive for media and leaves it in its current location. To do so, assuming that the media on your exteral HDD is currently not listed in your iTunes library:

• Run iTunes
• Select Edit > Preferences > Advanced (if necessary, press Ctrl-B to enable the menu bar)
• Make sure that the options to Keep iTunes Media folder organized and Copy files to iTunes Media folder ... are unchecked
• If you now use File > Add Folder to Library... and select the folder(s) containing your media on the external drive your songs will be added to the iTunes library without moving or copying them. The result of this is to add entries to iTunes' database containing links to the media files in their existing location.

However, although this approach "works" it results in a "split" library, where the iTunes database (iTunes Library.itl) is on the system drive and your media is elsewhere (possibly more than one location). Such a layout is difficult to back up and to migrate between computers. A much more robust solution is to have your media and your library database, artwork, other associated files in a standard folder structure on the external drive, looking like this:

Turingtest2's user tip on Make a split library portable for a detailed description of the preferred layout and how to achieve it. On the system I'm using right now I have exactly this setup - iTunes is installed on my C: drive (32GB SSD) but my library, in the layout illustrated above, is on a 1TB USB 3.0 drive (of which the library uses around 750GB).