Job Interview.. Help!

I'm going for an interview on Monday (finally somebody wants me!).

The problem is, I have literally not gone for any interviews in my entire life. Well, I don't really count the one with my current company as an interview which was, what, 5 and 1/2 years ago. I mean, Dexter practically asked these :-

"Do you know what we do?"
After I told him that I had read the company website, he took out this huge product catalogue file and started explaining each and every single product.

"Why did you study Engineering? Not many girls take that course."
Understandable that during my time, there were only 5 girls out of 150 plus students. But am sure by now, there are more females venturing into that field, aren't there?

"The R&D staff are all males. I'm trying to hire more females to balance it out."
What the??? Hence I got accepted without further question, started the job few days later and became the one and only female in the dept for almost half a year.

Now, does this count as a proper interview? You tell me. My friends used to tell me they had to undergo many personality tests, and they were bombarded with lots of technical questions. I guess I'm pretty lucky that way!

So, how should I prepare? What should I do? What do interviewers usually ask? What should I say? What should I ask about?

I've been contemplating to leave for a while now. I occasionally browse for job vacancies and try my luck. I guess either I'm just not qualified enough or coz the fact that I can only be made available after 2 months. So yeah, I'm really happy when I got the call for this interview. It's just that as much as I'm quite keen with this one (listed company, should be quite good, right?), I don't plan to leave the current job until at least after the wedding. Coz I still have a month's leave and my Long Service trip to take. Should I be honest and tell them that if I get accepted, I can only join them after 4 months time? Am sure even if they do accept me, they wouldn't want to wait for me for so long, right?

What should I do?