who i can choose rival factions?

You have to choose one in the small window of time BETWEEN missions. You should be able to choose one on June 1.

I was just able to join one of the Factions, about 2 minutes ago. You have to go to http://crossfire.z8games.com/events_gc_choose_side.aspx and choose which side. Then you'll be taken to the clan page of that Faction. You'll see the "Apply" button on the clan page. Click the "Apply" button of the Faction Clan you want to join, it'll ask you again if you're sure you want to join and you click "Apply" or whatever to confirm your decision, and you'll be signed up.

Joining a Rival Faction Clan does not affect your membership in your regular clan. Once you join a Rival Faction, you cannot change your mind and join the other. Also remember that the Ribbon for joining a Faction may not appear on your Profile for 24-48 hours.