I can give you some ideas to get you started on your comparison of Rosalind and Celia that you can use along with your own thoughts about the two characters.
Celia and Rosalind are, of course, cousins - both privileged, much-loved only daughters of men who are leaders. Though Celia's father has usurped the position of Rosalind's father, banishing him to the forest of Arden, she and Rosalind are still very close friends, almost like sisters. They are both loyal to their fathers, though Celia has not really come to terms with what her father has done to her uncle. Where they differ, however, is in the way that they express themselves. Rosalind is a leader; she is independent, stubborn, and intelligent. She knows what she wants, and she is not afraid to pursue it. When her uncle banishes her, she immediately decides to disguise herself as a boy and go find her father.
Celia is more of a follower. She is a little bit ditzy, but very sweet and easygoing. She decides to go with Rosalind, dressed as a lower-class young woman, but Rosalind is the one who is clearly in charge. Celia has her own kind of bravery in that she is willing to defy her father and go with her cousin, but she does let Rosalind take the lead. The two young women are great foils for each other; their personalities work well together. Celia's caution balances Rosalind's hot-headedness.
If you use this response in your own work, it must be cited as an expert answer from eNotes. All expert answers on eNotes are indexed by Google and other search engines. Your teacher will easily be able to find this answer if you claim it as your own.