1) If I declared a war with someone, should it be accepted by the others first?
2) If i created an alliance with someone, and my alliance win, but, since most diplomacy gametype are FFA, will we get exp when the game were voted to end?
3) If i declared a war/alliance with someone, can i un-declare it?

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*with common diplomacy rule

Everyone has slightly different rules but generally:

1) No, you just declare war, if it is multi-day be sure to wait one turn.

2) Vote-to-end ends the game with no points awarded, so I suggest when your alliance has "won" (but really no one has won yet) you agree with all of your alliance members to have a free for all, or just have the weakest members of the alliance surrender. That way someone will get the well deserved points.

3) You can break alliances at any time, but you usually need to wait a few turns before declaring war on a former ally. War cannot be un-declared, but you can agree to peace if both players agree.

With typical diplomacy rules:
1) You declare war following the game creator's instructions on how to do it legally. Noone must accept it.
Typical legal declarations are before the 2 min in real - time and 2 days in multi-day. In multi-day you also wait a turn before you launch attacks.
2) The point of a diplomacy is not to win and gain points but just to have fun. If you want to win, you should go play other types of FFAs.
3) You can always leave/break an alliance, but it is customary to leave it at least one turn apart from declaring war.
You cannot take back a declaration unless the declared nation agrees to peace.