How to design the A-B amplifier? I mean I have understood the working but how do i choose the values?
To start with I propose to feed the transistors not from in between the two biasing diodes thereby preventing signal distortion due to the diode properties.
This is only a little change and does not influence on the calculations. The -VCC is not marked in the picture. 



First a simple verification of the requirements and the limits of the transistors. 

You have a supply voltage of 12 Volt. Consider the input signal as a simple sinewave. The peak value of the sine wave can not be more then 12V. Forget about the little details for the moment. 

So the RMS value is more or less 8,5V 
Apart from the transistor limitations and other details your power limits already at 9W.

Now we look at the transistors.

Look in the datasheet for the max ratings of the transistors. From there you can calculate the max. allowable power and transistor dissipation. Keep a close eye on the limits. Max current is mostly to much your max power is also to be taken into consideration. 

From here try first and come back with questions showing what you have done.

Taking a look at the new schematic. 

First of all you are making it more difficult for yourself than you should.
Keep the output transistors NPN / PNP whith the emitors directing towards the load. Also for now do away with the output capacitor and use a symetric input voltage. Keep the system symetrical. And then look at the simulated output again and find the limitations.

Taking a look at the comments. 

Read the comments and respond to them by updating your question. Do not remove history. Show the progress.