Must have a BA law degree and get bar exam depending on the stae you are living.

You must go to law school which is three years after you get a undergraduate degree. Then you would take classes in criminal, constitutional, and immigration law. Upon graduation you would need to take and pass the state's bar exam. You would then try to work for an immigration firm or start your practice. I hope this answers your question. Thank you.

There is no "way" to become an immigration attorney. As long as you are already an attorney (undergraduate degree, law school degree, and pass a state bar test [in any state]) you then simply decide to practice immigration law. There are no further qualifications or restrictions necessary to become an immigration attorney.

Technically, since immigration is federal administrative law, anyone licensed to practice law in at least one state may practice immigration - and call themselves an "immigration attorney."

But, to actually be any good, it makes sense to work for at least a few years with a senior/experienced immigration attorney. There are law school courses in immigration at some schools, but really the only way to become good is supervised practice.