I'm often asked how to join a lab. The following is what I replied to one student. I'm pasting it here reference by others, modification / updating, and ease of sharing with other students interested in research.
Finding a lab to work with involves first identifying a lab that is a good 'fit' and then contacting them and making the case for why it makes sense for both of you (you and the lab) to work together.
Finding a lab that is a good fit. Doing this carefully matters. By analogy, though any restaurant serves food, not every restaurant is a good fit for a particular occasion (e.g., would you go to Wendy's for a first date?). Take the time to understand the categories of labs and then the specific labs within that category.
To identify the category, I recommend using the parts of the Psychological and Brain Sciences website aimed at prospective graduate students. This is where, after all, we showcase the ongoing research in the department in the most student-centric way. The different 'PhD specializations' represent different flavors of research going on in the department. For each area there is information about what type of research faculty / labs in this group do and the requirements for the degree. Clearly you can ignore the degree requirements part. You should focus on whether the research described is of interest to you. I recommend reading these descriptions for at least 3 areas before settling that you know which is the best fit-there is a lot of overlap and you may be surprised to learn that your first guess is not the one that sounds most interesting.
Once you find the area/category that best aligns with your interests, clicking on the 'meet our faculty' button will take you to a roster of faculty who do related work. Some faculty are listed under multiple areas. Clicking on faculty members' names will take you to a page that will allow you to learn more about their specific work. Again, be sure to read at least 3 (but 6 or more would be better) before deciding you know who would be a good fit. It isn't expected that you understand everything you read. You are just looking for evidence that the kind of work they are doing aligns with your interests. Some questions to consider - In their work, do they focus on humans, animals, cells, or computer simulations? Is that what you want to do? Do you think that the questions they are asking and interesting and/or important? It is okay to identify several that are good fits but pursue them one at a time.
How to pursue joining a lab. The next step is to send an email introducing yourself and asking to join their group as a researcher. You can run a draft by me before you send it. You can use the following three paragraph format. Keep each paragraph to 2-3 sentences (4max):
Why you are writing to them, include specific details as to why them in particular. Cite specific evidence (what information did you learn about them or their research that causes you to write).
Who you are. Include your year, your long term goals, how working with them fits into those goals, any relevant prior experiences you have had.
What you are looking for. Indicate that you are looking to help their research to learn X Y Z, when you are able to work (summers or semesters or both) indicate interest in building a long term relationship and if you hope to build towards an honors thesis or to help publish a paper