What to pass in Transaction Input using Bitcoinj library

URGENT – Need help with setting up Transaction Input and Output objects

Transaction rawTransaction = new Transaction(params);

txnAmount = Coin.valueOf(50000000);

rawTransaction.addOutput(txnAmount, toAddress);

txFee = Coin.valueOf(12800);

amountToReturn = balance.subtract(txnAmount);

amountToReturn = amountToReturn.subtract(txFee);

rawTransaction.addOutput(amountToReturn, fromAddress);

I am able to setup transaction outputs but not able to figure out how to set transaction inputs for fee

Currently with above code the signed transaction is having 0 fee