Hi
Best to read up on what you are interested in, and just show a passion for what you believe in. If your given written tests make sure you know how to write in academic essay style e.g. intro,body,conclusion.
Reading up on your specified area is always a bonus, if you go in saying i want to be a social work because i want to help people you very unlikely to get in they want more than that, read the definitions of what social work is all about, because there is a lot more to it than just helping people.
Interviews are scary, and it is dependant on the uni you apply what to expect, some i went to expected presentations, some wanted written assignments the one i got into was written test and interview quite easy, i just made sure my commitment came through. They need to know you are dedicated to finishing the course as well as places are limited. They are unlikely to give the place to someone who does not show they are dedicated.
Good luck to anyone going through this process, its not easy but worth the effort of a little study before you go.