The play opens with three of our main characters, LEONATO, his daughter HERO and her cousin BEATRICE getting news from a MESSENGER. The information the messenger provides sets up the scene: the prince and his lords are returning from a battle in which they have had great success. Once LEONATO has found out that all is well, and that a young nobleman named CLAUDIO has made a name for himself, the rest of this part of the scene is dominated by BEATRICE. She demonstrates her vivacious wit by making fun of BENEDICK, one of the returning lords who we are told she has an ongoing "merry war" with. The names BEATRICE and BENEDICK correspond - Beatrice means 'she who blesses' and Benedick means 'he who is blessed' which the contemporary audience would have spotted as a sign that the two are destined to be lovers despite her apparent low opinion of him (think modern soaps where characters who display passionate dislike often end up in bed together the next week).
The prince DON PEDRO, his half brother DON JOHN, CLAUDIO and BENEDICK arrive and LEONATO and the prince exchange greetings and pleasantries. They display mutual respect - LEONATO naturally respects his prince and DON PEDRO is grateful to LEONATO for hosting him and his men. This brief exchange is barely over when BEATRICE and BENEDICK start criticising each other and expressing how little they each care for the other. They both have razor sharp wit and use it to insult each other. Such vehement dislike and clever turn of phrase is rather funny, considering they have such strong passion for each other and are the only ones who cannot see it.
All but BENEDICK and CLAUDIO leave. Immediately CLAUDIO asks BENEDICK's opinion of HERO. BENEDICK reels off a list of reasons why HERO is inferior: she is not fair-skinned enough to be desirable and is far too short for his liking (tall women were believed at the time to be more likely to survive childbirth thus make more suitable wives), even BEATRICE is more attractive than HERO in his opinion. BENEDICK goes on to despair that no man can seem to stay a bachelor anymore and vows that he himself will never "turn husband." CLAUDIO doesn't take him too seriously; he is already deeply in love. DON PEDRO returns and joins their conversation, agreeing with CLAUDIO that HERO is a worthy woman. DON PEDRO even challenges BENEDICK's strong views, saying that he will see him sick with love before too long (predicting what we all know will happen). BENEDICK leaves and DON PEDRO agrees to help CLAUDIO win over HERO and the approval of LEONATO by pretending to be him at the disguise party.
ANTONIO is telling LEONATO that one of his servants overheard DON PEDRO talking to CLAUDIO about his plans to woo HERO. As we know from having witnessed this scene the servant has got it wrong; DON PEDRO was offering to help CLAUDIO by pretending to be him while talking to HERO and LEONATO - not actually wooing HERO for himself. Unfortunately LEONATO is pleased with this news as marrying his daughter to a prince would be a great honour.
DON JOHN is miserable: he has been accepted into his brother's good favour but as an illigitimate son he has no claim to power or honour. As such he seeks to deliberately make mischief (he has quite a chip on his shoulder). CONRADE tries to cheer him up with little success, but when BORACHIO enters and and informs him of DON PEDRO's plan to woo HERO for CLAUDIO he perks up, spotting an opportunity to make trouble. When he gets his men to swear their loyalty it is reminiscent of troublesome teenagers promising not to tell on each other before commiting their misdeeds. DON JOHN is entirely selfish, but sees himself as the victim of prejuduce even though in the play the other characters treat him respectfully.