Do you know what research assistant do at law school ?

Hello zarka,

The simplest answer to your question is: whatever the professor you're working for wants you to do.

I was a reasearch assistant for all three years of my JD and did everything from proofreding books and articles, to researching legal topics, to taking photographs for my boss, to categorizing and putting all of his books and journals into order, etc.

It was great because it covered my tuition but was a bit hard when the research piled up on top of your daily classes.
Hope that helps,
Droit

Is a job like that pretty easy to get?

No, not easy to get. I was lucky in that I was interested in a very specific program that the professor I worked for was in charge of. Otherwise, I doubt I would have been given the position.