Thinking when you don't know openings

Could anyone please tell me how to think if you do not know the opening and show me some examples?

It's sort of a broad question. People usually tackle topics like that in books. 

Your rating implies that you may be familiar with some of the more basic ones already.

Have you read any books on opening principles?  

You have to spend time and think about your moves more. When someone plays an opening I've never seen, I spend 2-3 minutes trying to figure out why the move isn't seen often and try to take advantage of it.

How do you take advantage of it in the game above?

Following the Opening Principles:

Control the center squares - d4-e4-d5-e5

Develop towards the center

Castle 

Connect your rooks

Thank you that helps a lot

Keeping the opening simple is easy :-)

It's a good thing you gave a specific example since with such a general question I would have talked about a lot of generalities without addressing your main concerns if I had responded earlier. In this particular example, here are some warnings that Black was doing something wrong:

(1)

3...Nd7. Minor squabble: usually such B-QN5+ bishop checks are blocked with ...B-Q2, though it is partly a matter of taste and intent. (http://www.chess.com/forum/view/chess-openings/why-block-a-bishops-check-with-another-bishop)

(2)

8...g6. The normal move in the Sicilian against Bg5, is ...e6. That prevents doubled pawns (that should have been forseen) and simultaneous creation of an isolated d-pawn (that also should have been forseen). This in general is largely a matter of knowing typical Sicilian moves, since Black's d6-pawn is often weak, one reason I advocate learning the ideas behind the openings early on when starting to play chess.

Thank you, that helps a lot.

The key in unfamiliar openings is to try to play simple solid moves (often, they are developing moves), to decrease the risk of playing a move that turns out to be totally bad. In this game, that means playing 3... Bd7 instead of the more sophisticated 3... Nd7, and 5... Ngf6 instead of 5... a6, because after this Black falls behind in development, which is always dangerous. Then 8... e6 makes sense, but the position is already unpleasant and in this case using normal means is not enough anymore (strong GMs have tried 8... h6 and 8... Bc6 there instead).

Also a good amount of time should be devoted to check the possibilities by the opponent. Never play a move without asking yourself what he will do after it.