Does it easy to make electronic speed controller , if yes then how to make it .
Which components are required.

Easy to just make if you have Some coding skills and good soldering skills. but the parts are costlier than buying one. also quality, response make things harder.

What kind of ESC are you talking about?
For BLDC Or Brushed motors?
Micro or Normal?
Power Output?

Normally An ESC contains these parts:
 FET + Micro Controller (usually Atmega8u) + Capacitors + 5V Voltage regulator(usually LM 78L05)

Regulator not for BEC, thats for supply power  to Micro Controller.

for bldc motor and esc can generate 20A current.
i have atmega8L microcontroller, can i use it for esc designing.

ESC does not generate any current. The current is pulled by the motor from the battery. When you say a ESC is 20A it means it can handle 20A without burning any of the FET

which fet should i have to use

which FET is suitable for 20A current supply
 

This may help you
http://www.rcgroups.com/forums/showthread.php?t=1679621

thank u, i m on it

Use "D206" single one can handle upto 15 amps, so use two in parellel per channel,

Total 6 Fets for single 30A ESC

can u give another code of n and p fet

"FDD8796"

only n mosfet can work?

Yup.

I want to make 30A esc ,
In this ckt. Which type of mosfets r used,

In Some forum it says that upper part is p-fet and lower is n-fet, wht to do .
One thing more , when esc is on then atmega8 will supply continuous signal to rotate the rotor?

I can't see the circuits well, anyway we can do mixed configuration and single type configuration.
Atmega8 produces pulse sequence to FETs not continues voltages, that sequence will go to through transister will give a amplified out to drive FETs.

Does atmega8 continuously give signal to transistors ? If yes than it can be damage In short time .
And fets  used should be n type or both
 n and p type