As in most "general" queries these days, Google is the place to start:
Prepare for interview[
^] - there is plenty of advice there, most of which is summarised by Peter..."Relax, sleep well, eat good, tell only truth..." - either you know your subject or you don't: if you know it, then show you know it. If you don't, you don't stand a chance.
So relax, and prove what you can do.