with the use of a transformer, bridge rectifier, and a voltage regulator (+ a few other components, such as a fuse, etc.)..... or buy a wall wart that will give you the proper voltage output that you need.... much easier and safer for beginners.....
As BMorse said, "wall warts" or plug supplies are a great way for beginners to get a safe and inexpensive power supply started. Many are not regulated. If you find one that's rated for 9v-12v at 500mA to 1A, you can add a couple of capacitors and a 7805 fixed voltage regulator with a heat sink attached to make a very cheap 5v regulated supply. It won't be terribly efficient, but it will work.
.hi I am a fresh student in electronic n i need help for this project my prof. is planning steping down rectifying a 240v ac to 5 or 6v dc. i need to know the components and schmatics. Thanks