In Act I, Shakespeare has introduced the principle characters and the problems that face them. The principal protagonists are Rosalind and Orlando, with Celia as a secondary protagonist. The conclusion of Act I sets up their response to their problem.
The principal antagonist, from whom the worst troubles stem, is Duke Frederick. He sent his brother and ruler, and Rosalind's father, Duke Senior, into exile by usurping the throne. Oliver, Orlando's brother, is the secondary antagonist without whom a primary subplot couldn't exist: Oliver hates Orlando, for reasons unknown to us, and seeks to kill him.
The problems Shakespeare introduces are Rosalind's rejection and exile by Duke Frederick; Oliver's malicious intent toward Orlando; and Duke Frederick's sudden enmity toward Orlando.
The complications to the problems are that (1) Celia insists on renouncing her father and fleeing with Rosalind, bringing Touchstone with them as they flee to Arden Forest and that (2) Orlando, being warned by Lord Le Beau, fears for his life from Oliver's enmity and Frederick's.
I would thou hadst been son to some man else:
I did find him still mine enemy:
... thou art a gallant youth:
I would thou hadst told me of another father.
The conclusion of Act I positions the two antagonists as mortal enemies of the protagonists. The conclusion has Rosalind and Celia planning to take up false identities, Rosalind/Ganymede as a man and Celia/Aliena as a lowly peasant girl dependent upon her male companion, Ganymede (both girls took their jewels and other wealth with them so the disguises are additionally critical to their safety). Act II will open with the discovery of the girls' flight, while Orlando's fates wait until Act III for more illumination.