What should I focus on when choosing what field of psychology I want to go into?

I'm pleased to hear that you are aware of the many different sub-specialties in the field of psychology! Too many choices, however, can be overwhelming, however.

I have three suggestions to help you to focus (in addition to what Mr. Keemer, above response has said). First, take as many different courses in the discipline of psychology that your college/university offers. Second, develop relationships with the faculty members at your institution so that you can volunteer, at first, in one of their research laboratories to see "what fits" you the best; student volunteers may be offered a part-time job after successful volunteering with a faculty member. It can be helpful to rule-out areas that really do not excite you, or just leave you cold, so to speak. So, between the coursework and then working with a faculty member or two, you should be able to narrow your focus. THEN, thirdly find an internship (either during the academic year if you have time) or during the summer - again for the purpose of finding out what excites you the most AND what you are good at!!!

Don't be overwhelmed by all of the choices. After you've ruled-out specialties that you find are just dull or boring, work toward developing your skills in the specialty(s) that you find the most interesting by working with a faculty member at your institution.

Consider the population or aspect of psychology that interest you the most. For instance, if you are interested in working with children, you may choose to focus on child psychology. If you are interested in how the brain impacts physical health, you may choose to specialize in health psychology.
One of my areas of interest was working with court-ordered client. I wanted to know and understand how their trauma impacted their crimes. As a result, I chose to focus on forensic psychology.
This is an excellent question and it is great that you are interested in psychology.

This is simply a matter of personal preference and no one other than you can answer that question. I would look at a degree program and specializations within that major and see what looks like it might interest you.