What are topic sentences I can write about the movie Finding Nemo?

I think by "topic sentence" the question might be referring to a "thesis statement." A thesis statement is an argumentative statement that is made by a writer about a particular topic. I usually recommend that students write thesis statements about story themes or characters. Keep in mind that a thesis statement makes an argument that the paper intends to prove. It isn't a statement of fact, because then there is no argument to be made.
For the movie Finding Nemo, I like thesis statements that push readers to think more about Marlin than Nemo. For example: "Although many young viewers cheer the accomplishments of Nemo, the real hero praise should be entirely focused on Marlin."
If that sounds too negative of a thesis, then write a thesis that focuses on the friendship between Dory and Marlin.  "While Finding Nemo has plenty of heroic moments in the film, the real message of the story is the importance of developing strong friendships and supporting those relationships no matter what."