You going to have to go thru and tag them with the appropriate metadata for album and artist.
There are paid solutions to help with this such as TuneUp Tagalious but you can do it yourself for free. Select a group of songs from the same album and artist, right click, select get info and fill in the appropriate fields.
A couple of thoughts... First of all, I used TuneUp a few years back and it absolutely wrecked my iTunes library. Mine was out of control and needed help, but IMHO, TuneUp made things worse.
As Karen says, manually tagging the meta data is free (money), but can be expensive (time). As with most issues fixing problems always costs something - time or money. But I digress.
Also, as Karen says, simply editing the meta data (right click, get info, edit, save) is the way to go. The good part is that you can select multiple tracks and edit the common info in one motion (Album title, Artist, Album Artist, CD #, Genre, etc). That makes manual editing somewhat less painful.
Good luck, and happy editing!