Do we have to stand in set positions, and who serves next?

Positions (Rule 7.4 and 7.5): there are no set positions in beach volleyball, and no positional faults. At the moment of the serve, both players must simply be inside their own court (except the server). Otherwise you may stand wherever you like.

Service order (Rule 7.6 and 12.2):

• The team captain sets the service order straight after the toss, and it must be kept for the whole set.
• If your team wins a rally while serving, the same player serves again.
• When you win back the right to serve, you "rotate": the player who did not serve last time serves.
• Serving out of order is a service order fault: the opponent gets a point and the serve.

The toss (Rule 7.1): before the match, the winner of the toss chooses either to serve or receive, or which side of the court to start on. The loser gets the other choice. A new toss is held before a deciding 3rd set.