Piano music numbers the fingers the same way on both hands: the thumb is 1, the pointer 2, the middle finger 3, the ring finger 4 and the little finger 5.
Put your right thumb on middle C and let the other fingers rest on the next white keys: 1 on C, 2 on D, 3 on E, 4 on F, 5 on G. This is the C position, and a lot of first tunes fit in it without moving your hand at all. The left hand's C position is a mirror: little finger (5) on the C below middle C, then 4 on D, 3 on E, 2 on F and the thumb on G.
Curve your fingers as if you're holding a ball, play on the fingertips, and keep your wrist level. Press each key down fully, then let it come back up before the next one.
Practice
Five fingers, right hand
Right hand in the C position: play up to G and back down. The number under each note is the finger.
C3DEFGABAC4SDDEFFGGHAJBKC5WETYU
Computer keys: ASD… for the white keys, WET… for the black keys, ZX move an octave.
More info
Tips
Play the five-finger exercise slowly with each hand every day for a week. Even notes matter more than speed.
Playing the exercises
Use a MIDI keyboard (Chrome, Edge, Firefox or Opera), the computer keys (ASD… for white keys, WET… for black keys, ZX to move an octave), or tap the keys on the screen. A key lights up after a couple of wrong tries.
The answers
Which finger is number 1? The thumb. Both hands count from the thumb: thumb 1, pointer 2, middle 3, ring 4, little finger 5.
In the right hand's C position, which finger plays G? 5. C D E F G are fingers 1 2 3 4 5, so G is the little finger.
In the left hand's C position, which finger plays the C? 5, the little finger. The left hand mirrors the right: the little finger is on C and the thumb on G.
How should your fingers sit on the keys? Curved, playing on the fingertips. Curved fingers on their tips are stronger and more even than flat ones.