Hi Sumanta,
1) pin3, contrast control
You can connect the variable of a preset not less than 10K to pin3. Other two pins of preset should be connected to VCC and GND.
5) pin15, anode (A) of backlight led.
You can give GND here
6) pin16, cathode (K) of backlight led.
You can give VCC directly to this pin (because in the datasheet an internal resistance is shown), but better through a resistor not less than 1K.
Answers for 2), 3), 4)
I’ve searched a few minutes, but couldn’t find a good datasheet of your lcd. Do one thing, try to find out which is the controller used in the lcd, and search for the datasheet of the controller. You will get a lot of informations.
If your lcd have commonly found KS0070B, then all the answers to the questions 2, 3, and 4 are in the following timing diagram. Try to understand it properly