You can create a custom playlist, name it whatever you want it called, then drag & drop the songs from the main library over to the playlist. When you're ready to burn click the burn button on window's far right. BTW you should check your burn settings before you burn a cd. Go to iTunes menu bar, Edit menu>Preferences>Burning or if you have the new iTunes version 5 then you will need to select Advanced tab>Burning & select whether you want to burn an audio cd or an mp3 disk, etc.

For a 90's playlist, the song needs to have its year included in the id3 tag. You can add them manually if it doesn't have one. Simply select the song in the library, hold down control & i, then add the year under the info tab section. The 90's playlist is what's called a Smart Playlist & can be identified by its colour (purple instead of blue.) It means songs will automatically go to that playlist if they fit the criteria for that smartlist which in this example is being from the 1990's. You can set up your own smartplaylists using criteria such as artist, year, album, etc. Or just use a normal playlist.