Other chords that you can use in this exercise are: A, E ,D, C, G, F, Am, Dm, Em, Fmaj7, Am7, Dm7, Em7, A7, D7, B7 and E7.
Don't worry if you're not doing this right. Just get used to holding notes with your voice whilst strumming the chords. You can always record yourself to hear if your singing is in tune.
Do both exercises for 5-7 minutes before you start practising your song.