Have you looked at your law school's particular requirements for a First-class answer? I suggest that as a good place to start as you can see what sort of answers you need to provide in exams etc that would point an examiner towards you achieving First class marks. Usually, they talk about what sort of sources you need to draw on and, importantly, how confident you come across as really knowing a particular area of the law - better still than someone who writes a strong upper second class response.
In my first year, my tutor asked me at the beginning of the year what mark I was aiming for in my first year exams. I replied: 'As this is First year, I think I am aiming for a First'. I don't really understand what I meant then and I don't think my tutor did either! When I think now what my response should have been, it is: 'I want to achieve the best grade possible for me'.
Its great that you are aiming for a First, I too am aiming for a First and I'm in my third and final year of my law degree. The difference between me now and then, is that now I genuinely mean it when I say it and also, with the benefit of studying law for two years (and being able to judge better my own capabilities) its more of a realistic challenge.
Part of the challenge is wanting it in the first place and a good man once said that its good to set the bar really high and work towards fulfilling that goal because, even if you fail, you still end up a lot better off than if you didn't 'go for it'.