How do I change the bit rate of my songs on iTunes?

I want to change them to 128 bit rate so I can fit more songs onto an mp3 cd, but how do I change the bitrate? Im very new to iTunes, and have no idea what im doing so I need some help please lol.How do I change the bit rate of my songs on iTunes?
Right click on the song you want to become 128 bit. Then click Create AAC Version. The song will start to convert. About half a minute later you'll get the song fully converted into 128 bit.

To check to see if the song is in 128 bitrate right click on the song and click Get Info. A new window should pop up telling you the information on that song, somewhere in there is the bitrate.