I live in Massachusetts, USA, as an international student with F1 visa. I did a 14-week internship in California last year from May 26, 2015 to September 4, 2015. Do I need to fill some income tax return form in California?
The short answer, yes.
The relevant information can be found on this page. Excerpt:
Determine your residency status: Residents, nonresidents, and part-year residents
A part-year resident is any individual who is a California resident for part of the year and a nonresident for part of the year.
How California taxes residents, nonresidents, and part-year residents
Part-year residents of California - Taxed on all income received while a resident and only on income from California sources while a nonresident.
To register, go on this page:
And once you are done registering: