Get started early in conceiving of your project and seeking a potential advisor.
It is in your interest to conceive of a research project and seek out a potential advisor by the end of your junior year and/or during the following summer, so that you can be ready to start actual work on the thesis at the start of your senior year (or even before that, e.g. over the preceding summer). The entire Linguistics faculty will then decide collectively whether or not to accept you to the thesis program at the beginning of your senior year.
If your proposal is not accepted, you still have the option of writing up your research and ideas in a one-semester independent study (LING 98a/98b), under the guidance of a faculty advisor.
Be sure to apply for IRB permission (where required) well in advance, so that you can use your data toward a thesis.
It is your responsibility to determine, in consultation with your faculty advisor, whether or not you need to obtain IRB (Institutional Review Board) approval for the data which you will collect, in order to be able to use the data in your thesis. This is typically necessary for projects which involve data gathered from human subjects, e.g. through interviews, participant observation, databases, etc.
If such approval is necessary in your case, IRB approval needs to be obtained before any experiments or other data collection begins. Some students in this category apply for IRB permission in or at the end of their junior year; others apply first thing in their senior year.
The IRB's purpose is to make sure that you do not take advantage of your research subjects, and that the research benefits outweigh the risks. IRB approval takes time, and you need approval before beginning your study, so submit your proposal as soon as you have a topic in hand to avoid any unnecessary delays in your research.
An explanation of the forms you need to submit can be found on the Office of Research Administration website.
Note that the review form requires a description of your research, including a detailed description of methodology, attached to the form.
If you’re applying for an exempt or expedited review, put your completed application into a pdf document and submit it electronically. If you’re applying for the full review, you should additionally hand deliver the completed application to the Office of Research Administration in Bernstein-Marcus 121. If you’re applying for a full review, pay attention to application submission deadlines, since all the committee members must read a very large volume of applications. Take care of this in September if at all possible or you may lose valuable time in your research. Students must also participate in a training session through the CITI program.
Please note, sometimes applications aren't approved and no notification goes out to you. Be sure you follow up on the status after the meeting in case you have more work to do.