How to avoid this???

Keep cool, black has nothing on you. Continue with Nh2 and then exchange the queens with Qf3 or Qg4. You have a won endgame.

Let him throw away a piece for a queen-only attack, why not?

Off topic, but in the Italian game, after 3... h6 I've been told that 4. d4 is a good move.

His queen can do nothing alone. Being up a pawn, just continue like normal until he supports his attack with more pieces. Eventually you could try to kick his queen out with a knight of bishop, or trade queens.

You resigned a, most likely, winning position.

I hope you DID resign, because it is illegal on this site to ask humans for help in an ongoing game. Of course it is OK afte the game is finished.

ouch, a bit of calm please

It's illegal to ask computers, cats, dogs, and rodents as well...

 Did you read his post? He clearly says "I played white and resigned after that move."

Dammit. I DO sometimes consult with my cat. She just gives me a bored look and sometimes provides a big yawn and goes back to sleep.

Ah, the bishop sacrifice! I remember when me and my friend first found out about it, for a long time we both tried to lure each other into allowing the sac on h3 (or h6). Problem for black is that it takes some time to get pawns rolling toward white king, white has done nothing wrong so far so he must be having an advantage.

You should be playing games like this out, even if you think you are beat.  You weren't in this case, but even if you were, it's good exercise.  Give yourself a chance to win and give your friend a chance to blunder.

Yeah, resigning a game just because you don't know how to continue is just crazy if you are clearly nowhere near lost (in this case winning easily in fact).