How to add bitcoins in my wallet? I'm new to this BTC world, one day someone offered me to pay using BTC and made me create a wallet on https://multibit.org

I recently used it to pay for something online, and another transaction and another one ...

Now, my wallet is almost empty and I need more BTC to make more purchase so the question is : How to add bitcoins in my wallet ? *would like to use my credit card

There's only send/receive options in there :)

Thanks for showing me the way
Without knowing what country you are in it's hard to answer fully.

But, as Nick said, you can use Circle to purchase Bitcoins for USD using a credit card.

To further elaborate, Bitcoins are a currency, so the only real way to get them is either Mining, Faucets, or by converting your government currency into Bitcoin using an Exchange.