how important is to know chess openings?

Is there any website where I can see them easily?

Are chess openings important? Cannot you just choose sensitive moves instead of knowing which piece you are supposed to move in the first 8/10 moves?

http://www.365chess.com/opening.php

This is a good resource. I would also have a couple of theory books explaining ideas and plans as well as just the moves. If you're playing the moves with no understanding of the opening's plan, then your memorization is pointless (which sometimes applies to me).

Whether this is necessary all really depends whether you're getting bad starts in games against people between -200 to +200 of your rank. If so, yes, you'll probably need to learn a few openings. Many people say endgame study is the most important, while I say they're all equal, but middlegame study and general strategic planning are the most understudied parts of chess by most.