I’m assuming you want a PhD in psychology.
If you want a PhD in clinical, counseling, or school psychology, there are master lists for those programs. The American Psychological Association has a list of APA-accredited programs in those fields on their website. You can find their search tool [here[/url].
If you want a PhD in one of the other subfields of psychology (social, cognitive, health, quantitative, industrial/organizational, developmental, community, some others I’m forgetting) then there’s no good master list. However, there are professional organizations that maintain lists of programs. For example, Division 38 of the APA - the health psychology division - [url=http://www.health-psych.org/LandingEducation.cfm]maintains a list](http://apps.apa.org/accredsearch/) of both clinical health psychology programs and regular (non-clinical) health psychology programs. Division 27, the Society for Community Research in Action, [has a list](http://www.scra27.org/what-we-do/education/academic-programs/) of community psychology programs. And the APA website has a [decent list](http://www.apa.org/research/tools/quantitative/) of quantitative psychology programs. Some of the other APA divisions might also have lists that pertain to the other subfields - you’ll have to check out the APA website and look up the division website.
You can use these lists plus a combination of your own research. One great way is to look at recent journal articles (I’d say 2011 and after) that are really interesting to you, see who wrote them, and try to track down where those professors are teaching and doing research. You can also do an Internet search - I found my PhD program, in public health/social psychology, by Googling (although I do not remember what I used as a search term).
BUT (and this is a big but)
If you don’t know what you want to do, graduate school should not be your next step.
Graduate school is a means to an end…you do a graduate degree because you want to do something specific. For example, you’d get a PhD in clinical psychology if you want to practice as a therapist and/or if you wanted to be a professor of clinical psychology somewhere at a college or university. You’d get a master’s in psychology - maybe - if you wanted to practice as a therapist and could get licensed in your state with that degree.
But you don’t just get one without knowing what you want to do with it. The reason for that is you make an investment of both time and money into the degree, and you want to be sure that you actually need that degree and not something else, particularly before you invest 5-7 years in a PhD.
So while you should still search for a research assistant position and investigate programs, spend some time thinking about what kind of career you’d want to have and what graduate degree you would need for that career.