The intro is basically these three chords. Listen to the song for the changes.
I think the chord with the ? might be an Fm?

Intro/Fill # 1            Fill # 2

  C    ?    C    G         C
--0----3----0----3-       -0----------- | 
--1----2----1----3-       -1----------- | 
--0----0----0----0-       -0-----0-0~2- | 
--2----2----2----0-       -2--0~2------ | 
--3----3----3----2-       -3----------- | 
--0----0----0----3-       -0----------- | 
Verse one

C             F              G        Am          Dm  G
Secret heart, what are you made of?  What are you so afraid of?
C             F           G     Am             Dm         G
Could it be three simple words? Or the fear of being overheard?
C fill #2             F          G      C --> Fill #1
What's wrong? Let her in on your secret heart

Verse two
C             F     G           Am            Dm     G
Secret heart why so mysterious? Why so sacred why so serious
C             F        G        Am            Dm          G
Maybe you're just acting tough. maybe you're just not man enough
C fill #2             F          G      C --> Fill #1
What's wrong? Let her in on your secret heart

Bridge
F                       G         C
This very secret you're trying to conceal
                 F           G        C
Is the very same one you're dying to reveal
F   C                G
Go tell her how you feel

---SOLO----


Verse three
C             F            G
Secret heart, come out and share it,
Am               Dm      G
This loneliness, few can bear it,
C              F          G
Could it have something to do with,
Am                 Dm            G           C --> fill #2
Admitting that you just can't go through it alone,
F                   G
Let her in on your secret
C
What's wrong
F
Let her in on

Your secret heart