In order to make a new song, click a ‘+’ button at the very bottom left and you will see this page.
Software instrument is for the people who does not have their own instruments to record.
Real instrument is for when you actually record with your OWN instrument.
This time, I will use software instrument.

After you click, you are all set to go.
Press record button below in order to record your own music.
OR
You can make your own music by using already made music through GarageBand.
Now, I will show you how to add music in loops.

Step 1: Click eye button at the below. (refer to above photo)

Step 2 : Click type of song(s) that you would like to make and choose music that you want.

Step 3 : After you hear it, click the music and drag in loops if you want.

If you wish to expand the length of dragged music, put your cursor on it until arrow sign shows up. When it shows up, drag it as much as you want.

Also, you can easily listen to what you want by clicking mute button as well.

Now, you will be done making music now!
Listen and enjoy the music you made through GarageBand.

(If you want to share your music on Youtube, go to iMovie and combine both recorded video and your music because you cannot upload the file of GarageBand.)