First of all, I'm not a lawyer.
Being legally separated is not the same as being divorced. It means that two people who are married are living apart and the legal separation describes the conditions of them living apart. Mostly it would involve how expenses are shared and how children are cared for. So, you would still be married even if you were legally separated. If that were the case there should be some written court order describing the terms of your living arrangement.
But it's possible that the legal separation process was not completed. Ask your husband if he submitted any papers to the court and which court he was dealing with. Check with that court.
Immigration law is a specialty practice of law, just like contracts law, personal injury law or constitutional law - if you want to be come an immigration attorney, you would get an undergraduate degree, then apply to a law school. The law schools in Texas, Arizona, Florida, and California are places (for obvious reasons) where "immigration law" tracks are popular and readily available.
Your choice of an undergraduate degree isn't as important as some people might tell you. It is NOT necessary to major in anything called "pre-law". What is more important is that you earn very good grades, as the better law schools are all highly selective when it comes to admissions. Sure, Yale accepts only 7.3% of its law school applicants, but even the University of Wyoming accepts only slightly more than 1/4 of the students who apply for admission.
You will want to focus on a major that gives you exposure to a broad range of disciplines - law schools like to see applicants with psychology, philosophy, government and business courses on their transcripts. Don't discount courses like mathematics, which you might not see as relevant - trust me, lawyers spend a lot of time working with numbers, and if you ever hope to manage your own practice, you'll need both business and math expertise.
Law schools also like to see themselves as training empathic people who want to serve and improve their communities - this means you should take advantage of opportunities to involve yourself in social programs, whether it's mentoring disadvantaged students, working with the hungry and homeless, job training, senior citizens, whatever. You should do these things throughout your college years, not just in high school.
Law school is usually a 3-year program, so you're looking at 4 years for an undergraduate, plus another 3 for the JD (juris doctorate). You'll need to take the Law School Admissions Test (LSAT) near the end of your junior year as an undergrad, so that you'll have your scores in hand when you apply to law school as a senior. It's also worth noting that many (but certainly not all) law school applicants take time off between their undergraduate degree and their law school application - some law schools place a premium on work experience, especially if it's at all related to the study or application of the law.
Law school is an intense graduate program, which will challenge you to think about things in a whole new way. Many schools use a somewhat unique training approach called "the Socratic method", where the emphasis is on self-discovery gained while pondering "deep" questions, rather than taking notes as your professors lecture. Some people love it, some people hate it, some universities find it old-fashioned and don't bother with it. You'll do best in law school if you like to think more than follow instructions.
Law school, like every other professional graduate program (medicine, dentistry, etc) is an expensive endeavor. There are plenty of loans available, but most law school students leave law school burdened with heavy debt. The most recent figures put the average debt around $54,000 for state university schools and $83,000 for private law schools. Unfortunately, these debt loads turn many students away from some of the lower-paying community service-type law practices.
I hope this helped you - good luck!