I just deleted my original answer now because I realised it was wrong.
If you have a wallet.dat file, then multibit classic is not the Bitcoin wallet you would have used. Chances are, you used the original Bitcoin software.
First you will need Python 2.7:
64 bit version
Download this private key extraction utility which will display the private keys for your wallet.dat
Download the zip and unzip.
Open the command line (windows key + r) type cmd then press enter. Change directory to the folder you downloaded ie:
python pywallet.py --dumpwallet
press enter and you should hopefully see a list of bitcoin addresses and private keys (which start with the number 5).
Once you have these private keys, open a wallet with blockchain.info
and once you're in, click on the Import/Export button and click 'Yes I know what I'm doing'.
Click on Import wallet in the left side column.
You can then add the private keys by copying and pasting the output from the command line.