How can I locate a specific person that is already in my tree?
If you added or edited the profile, look in "My Contributions." Once you find the profile, click the star to Follow the profile, to make it easier to find the next time.
If you have not added or edited the profile, if you have details, you can use Find to search by name. Or by using one of the tree views to find the profile.
If by "my tree" you mean your private space on FS, where all of the profiles you've marked as "living" reside, you can see a list by going to the main site, clicking the Family Tree menu at the top, and choosing My Contributions. On the resulting page, click the "Private People" tab.
If you're using "my tree" because you think FamilySearch has individual trees just like every other genealogy website you've ever used, then please read the Overview (under the Family Tree menu) and other introductory material.
If "my tree" is shorthand for "the people in the shared tree that I'm related to", then there are a few ways to somewhat limit your search to your relatives. If the person is an ancestor, you can use your fan chart (because out of FS's chart views, it goes out to the greatest number of generations) and your browser's search function. If you've made any edits to the profile in question, then you can go to My Contributions and search by name. Or, if you're often faced with trying to find a relative without recalling the specific relationship, then I suggest choosing one of the offline third-party software options that can synchronize with FamilySearch's Tree. Download your relatives into the software and then use its search features to find people.