I want to make a voice changer such that I shall speak in mic of computer and the speaker output I shall connect to my mobile phone. And using voice changer software in computer I shall change my voice. Now I have 2 questions.
1) Where should I connect the output leads in phone? i-e, where 2 pins of mic are connected or somewhere else?
2) Do I need to amplify the sound? if yes, Please tell me the circuit also for that I am a beginner in electronics.

 

You need an isolation circuit to connect audio from your computer to a phone. The isolation prevents hum. An audio transformer is usually used.

The output from your computer is at a high level. The input to the phone where there was a microphone is at a very low level so you need an attenuator after the transformer.
Maybe the transformer can have the correct turns ratio to also perform the attenuation.

 

Can u pl z help me making complete circuit. Please give me the exact value of transformer and other parts used.

 

Audio transformers were used 50 years ago in transistor portable radios. I don't know if they are still available.
I don't know what is the output level from your computer so just use a 1:1 transformer and a resistors attenuator.