"Is the song key is C Major or A Minor?"
This piece is in d dorian.
"How to find out if it's C Major or A Minor?"
There are no accidentals at the beginning of the staff which could apply to both C major or A minor. But apart from the diatonic scales, there are also modes, and this happens to be in d dorian mode.
The only way to really tell what key it is in is to listen, and try to discover what sounds as if it should be "the last note".
You can cheat a little though, and actually look at the last measure to see what chord you find there. In this case, it ends on a D major chord, indicating the tonal center would be on d. d dorian can be seen as a special case of the normal diatonic minor scale of d. This may seems strange that a piece which is in a kind of d minor scale ends on a d major chord, but this is not that strange. Sometimes that is called a "picardy third" (= major third in final tonic chord in an piece that is in a minor scale)
"I saw there is big "A" letter above the time signature, does it mean the song is in A Minor?"
No. This is a rehearsal mark - a label that identifies a particular section. You can use this as a recommendation to hook in when you're rehearsing the piece.
"There are two staffs, but it's not a grand staff as it seems. What does that two staffs mean?"
There is an accolade indicating the staffs form a unit. Most likely this score is originally for piano.
"If I want to play with my ocarina, is the top staff that I should play? so the bottom staff can be ignored?"
Yes, it seems the upper staff is the melody. The bottom staff is accompaniment. You wouldn't be able to play the bottom staff on a wind instrument (except, maybe bagpipes or organ) since it sports simultaneously sounding notes (chords).
"If there are two note symbols stacked together, what does it mean? For example, look at the part with red square below: "
It's two independent parts (voices) on one staff. The upper voice gets its beams up, the lower voice gets its beams down. You should pick one when playing the ocarina. I think it would make most sense to always pick the upper most voice when there are multiple parts in the measure.
If the voices are completely dependent, then you'll find multiple noteheads on the same beam. There's a lot of that in the lower staff.
"Some note symbols has number on top, for example like the first note (D) symbol has number "1" above, the fifth note (A) has number "5" above, some others doesn't have number at all. So what does the number mean?"
I believe this is fingering for piano.
"The four notes that has 1 2 3 1 number above seems doesn't sound good when played (see image below with red square), their existence just break the melody between high A and B note. "
That's because they are a second voice. The upper voice lets the a ring on while the lower voice plays this scale figure.