Is it possible to pursue a Masters in Psychology in USA after completing Bachelors in Social Work in Nepal?
Short Answer: Yes, it's technically possible.
Long Answer: It should technically be possible, but will depend on a variety of factors, although the country in which you received the degree shouldn't be an issue in your case.
*Does your coursework satisfy the requirements of the specific graduate school you are applying to?
*Does your academic performance meet the requirements of the specific graduate school you are applying to?
*Does your GRE score meet the requirements of the specific graduate school you are applying to, if required?
*Are your English proficiency test scores at the level required by the graduate school you are applying to, if required?
*Are you in a financial situation that will allow you to study in the US?
Depending on your situation, you may have to take additional courses at another institution, i.e. a community college, to satisfy basic requirements before being eligible to apply for certain graduate schools.
You will probably also need to have your transcript translated in to English, if your school does not provide it for you. Your recommendation letters will also need to be in English.
As far as for the costs of tuition, your graduate school may or may not provide financial assistance in various forms, including TA/RA positions or straight up stipends. Costs of living, on the other hand, are much less frequently provided for although they do still exist, and more often than not only for doctoral students as funding is limited. Be prepared to have to cover some, if not most or even all of the costs associated with studying in the US.
Remember that schools will differ in what they are looking for in potential applicants, especially international. I strongly recommend that you contact the specific graduate schools you are interested in and inquire directly, to see if they can help you figure things out.