So I have Bitcoins on my BTC wallet (Multibit) , and I was wondering what I should do or save Exactly so in case of a computer failure I can get my wallet & money back ? thank you
You need to back up your private keys. Print them on pare, QR code...
Or you can move all bitcoins to wallet with paper wallet support )like armory or electrum.
These wallets use single seed (strong pass phrase) from with you can generate all addresses and private keys.
So storing of this seed is your lifetime backup.