Choose any proof in the course you are trying to write for practice. However, the Proof Clinic won't help you with any work that is to be graded. Don't bring proofs from your assignments or tests.
For example, here is a long list of proof problems that do not require any knowledge of calculus, linear algebra, or advanced math. They are intended just to practice proof writing. Try to write proofs for a couple of them.
Visit a TA in the Proof Writing Clinic and show them your work. For location and schedule, see see the office hours tab.
The TA will give you feedback and advice on how to improve.
Notice that the Proof Clinic is intended to give you feedback on proofs you have written, but whose quality you are uncertain of. The Proof Clinician won't write the proofs for you.