Powersupply
Hossam,
Well, first you need to know what rating you require and for what purpose.
AC,or DC output
100 VA, 500 VA, 2 kVA ?
Primary mains voltage 110 / 220 Volts, secondary voltages 12 , 32 Volts etc.
Then you need to know what type of loads you put on it or just use it as a dc bench top supply with adjustable voltage and current.
A range 0 - 24 Volts dc with currents up to 3 amps is very versatile.
The basic layout is in general MAINS, isolator switch, fuse, transformer primary.
from secondary through bridge rectifier , inductor, capacitor, to series regulating transistor 2 N3055 or whatever.
driver cct and (overload foldback system) if required.
Have a look on google for linear power supplies and there will be some examples.
I can attach a schema but it will take some time because i have to put it through flickr.
Good luck and keep us posted how you go.
For a benchtop supply you can also look in surplus shops for a secondhand one