Now, you'll need to organize your library using iTunes' built-in function for that.
To do so, click on File -> Library -> Organize Library.
In the window that pops up, you'll see two checkboxes. Make sure that both are checked. Depending on what version of iTunes you're using, the exact language used for the checkboxes will be a little different, though the first one should be Consolidate Files and the second some variation of using iTunes Media organization.
When both boxes are checked, click OK. Your iTunes library will then be consolidated and organized. This should take just a few seconds.