You have to calculate Service Tax like this way - 

You have to charge 12% on your service amount, that is called service tax. 

After that you have to add Education CESS 2% on Service Tax amount. 

Again you have to add another 1% Additional CESS on Service Tax amount. 
So total Service Tax will be = [12% +{(2+1)% on 12%}] = 12.36%. 
For example, if you bill amount is Rs. 100/= only. 
Then you have to add (100x12.36%) Rs. 12.36 with your bill amount as total Service Tax. Now the total bill amount will be Rs. 112.36 (Rs. 100 + Rs. 12.36) only