How to import old wallet to Multibit? I had 2 Bitcoins on my old PC and got a virus so did a format.
But I also did a backup of my bitcoin wallet.
Now I want to sell these bitcoins but when I went to the site, the official client was different and very heavy. And the site was showing as first option Multibit.

So, I got Multibit. But I couldn't import the old .dat wallet.
Am I missing something? Is there any way to convert the wallet for Multibit?

Is this safe? 
https://blockchain.info/wallet/import-wallet
Yes, blockchain.info's wallet import is safe. It encrypts things in such a way that not even blockchain.info can access your private keys. You can make transfers from there (it's a sort of online wallet), so you don't even necessarily need to transfer them to MultiBit before wanting to sell them.

To quote a previous answer with instructions on importing and then exporting to MultiBit:


  
  Visit https://blockchain.info/wallet/import-wallet. Drag the Bitcoin-Qt wallet.dat file into the dotted area. Follow the
  instructions.
  Login to the new wallet. Under the Backup heading click download.
  Open multibit choose Tools -> Import Private Keys. Now import the wallet.aes.json file just downloaded.
  Delete the keys from the blockchain.info wallet.
  


If you don't trust blockchain.info (despite their claims), you could extract the data entirely offline using pywallet and then import it into MultiBit.

It should also be possible to dump the private keys from the official client without downloading the whole blockchain (which is the "very heavy" part of it). See How do I import private keys from Bitcoin-qt to Multibit client? for help with that.

Once you have access to your bitcoins again and want to sell them, you could sell them on LocalBitcoins. In the US, I can also recommend Coinbase.