1:What should be the first step to take while seating yourself in a car with automatic transmision?
The car would be on P, just switch on the car, push the brake pedal and then shift to R to reverse or D to drive (forward)
2:What is D and D3 Im assuming its drive mode? if it is correct than what is the difference?
When the cars is in D it would engage all it gears 1-4 for 7th gen civic and 1-5 for 8th gen. When in D3 it would engage only first 3 gears.
3:How to drive on steep hills as auto changes gears at low rpm?
7th gen civic gives you good control on gear changing, if you keep the accel pressed it shifts at higher RPM and if you keep light foot on accel it will shift up quickly.
4:How to use brakes as there is no clutch?
Just press them... LOL. no need for clutch the cars will not switch off so no worries. if the cars in D when you would lift your foot from brakes it would slowly move forward (very handy in bumper to bumper drive scenario).
5:Are these gears like D,D3,R,N the same in all auto cars or are they only in 2004 shape civic?
Most of them have same, some offer more options to +- sign in triptonic gears to manually shift up and down. But do not worry civics do not have them. 8th gen civic has P R N D D3 2 1 and 7th gen has P R N D D3 2.
Please not that 2 means only 2nd gear would be engaged it will not shift to any other gear, which might be troublesome going uphills with few people on board.
Here is simple tip, for first few weeks just think you car has these options
P to park
R to reverse
N to nuetral
D to drive forward
You can switch off the car in any gear you want but it will only start in N or P. Also if you want to remove the keys from ignition you have to put it in P