How can i control a mini servo via Serial Command only two buttons
it should be like 'w' to move 2 step forward and 's' to move 2 step backward.
if i hold w button it will move to 180' at once and if i hold s button it will move to 0' at once..
You will not be able to do exactly what you describe with the Serial monitor because it only sends the characters entered when Return is pressed. Other terminal emulators, PuTTY perhaps, can be set up to send characters as they are entered.
The Arduino would need to read the input and be able to differentiate between a series of the same keypresses over a short period of time (key held down) and a single keypress (key pressed once). So, you would need to count keypresses over a period of and take action based on the number received in that period.
The first thing to do is to get the continuous sending and receiving of characters working. What code have you got so far ?