You should start thinking about topics for your thesis prior to the semester you plan to start writing it. As you take English classes get to know the faculty teaching them, attend office hours, and ask questions in class. Tell them you are thinking about writing a thesis and what some of your potential topics might be. (Or, if you don’t have one yet, talk with them about how to develop a topic!)
Once have determined your likely topic area, check out the faculty listing to see who has interests that match yours and contact them. You don’t need to have a precise topic and thesis established, but having some ideas and having a little research done prior to meeting with a potential advisor can be helpful.
More information about writing a thesis is available on the Honors page (most of the guidelines for writing a thesis pertain to students taking ENGLISH 681/682 or 691/692). You will need to have an advisor before you can register for a thesis course. Thesis advisors must be Assistant, Associate, or full Professors; unfortunately, TA’s or adjunct instructors do not qualify to direct your thesis, though you can certainly talk about your work with them.