What is the dramatic effect of Act 1 in Much Ado About Nothing?
I wonder if you are referring to dramatic elements that are employed by Shakespeare in Act I. If so, consider these choices of the playwright:
Rather than including the war or any of the battle scenes in the play, Shakespeare uses a messengar to deliver news of the outcome. Beatrice jokes with the messenger, who announces that the friends of the group have been victorious and are safe. Shakespeare wants readers to know that the visitors are coming from battle, but doesn't want any of the seriousness of war to affect the mood of the play.
In Act II, Shakespeare has Antonio tells Leonato that his servant has overheard Don Pedro tell Claudio that he loved hero. Obviously this is wrong, and the servant has given bad information. But the dramatic technique here is that we do not see the servant telling Antonia this. We see Antonio tell Leonato this. This removal from the source of the information is important, because it introduces one of Shakespeare's themes: "noting". Noting is the overhearing and passing on of information. It causes many many problems for the characters. and so Shakespeare uses it as a device to underscore its importance.
Look to stage directions and consider how characters present information to see more examples of "dramatic technique".
By "dramatic effect," I am assuming you mean what opening conflict or events did Shakespeare write into Act I to get his audience interested. If that is the case, Much Ado starts quickly and grabs the audience right from the start. We immediately meet Beatrice and Benedick, who clearly enjoy verbal warfare against one another. We're also introduced to the character of Don John, Don Pedro's disgruntled half-brother who desires to mess with everyone's happiness, especially that of Claudio.
Check the links below for more information, and re-read Act I on your own. I think you'll enjoy reading the dialogue between these characters and seeing how Shakespeare used witty banter (particularly in Act I, scene i) to capture his audience's attention. Good luck!