K I've done some research in this topic so I have an answer or two.
a) You need to complete an undergraduate degree that is recognized by the North American university you want to study law at. There's a discrepancy between NA and the UK. An undergraduate degree here in North America is a stepping stone onto graduate school, which is what law school is considered over here. Law school here is an endpoint for a lot of people, not a stepping stone and definitely not something most people consider to do RIGHT out of high school. The age of a law school student will range from 21 (which is typically how old one would be following the completion of an undergraduate degree) to mid 50's - as long as you have an undergraduate degree, you're eligible to become a lawyer here in NA.
b) You need to write the LSAT. It's the "Law School Admissions Test" and most schools require it. It is made of 5 sections that test your ability to organize your logic and apply it to situations - you can't study for it, just like the LNAT, but you can sure as hell prepare for it. It's been around for decades, so there's a lot of prep courses on it.
c) Someone mentioned prior that you can just sit the bar and write it to become eligible to practice law, if you're applying with an English law degree. This is not quite the case, as certain admissions commitees will need to evaluate your undergrad degree and gauge your standing and knowledge in North American law. You MAY have to re-do all 3 years of the JD program here, or you may be able to take one or two courses, or you may be able to write the bar right away - it all depends on the committee and the school.