Unlike classes and other academic experiences, you usually need to do some work to seek out research opportunities. No matter what type of position you're looking for (course credit, summer, paid, volunteer, etc.), the process is basically the same. Your first step will be to figure out which professors are doing work that you find especially interesting. To help start this search at Yale, get to know what the professors in the Psychology department do and what research questions they're working on. The best way to learn more is to google different professors, read about their work, and think about what you would find interesting. Once you've learned more, you can simply email the professors you want to work with to ask if they have any research opportunities available. Note, however, that professors get lots of emails from interested students. This means that you're much more likely to actually be accepted for a research opportunity if you can demonstrate that you are truly informed about the work being done in the lab. Before contacting professors out of the blue, take some time to find out what they are working on, and read several of their recent papers. By taking the time and effort to find out about some of the details of the work, you'll demonstrate that you are committed to the relevant area of study and that you're not desperately mass-emailing professors (which, as you've probably guessed, most professors don't like very much).