Well, I'm not out yet, so I can't give you advice on trans issues in particular, (much of which will depend upon what stage you are in in transitioning, as well as where you live) but as someone who has hired people, I can give you some general pointers.
What to wear depends on what kind of job you are interviewing for. Take a look at what people are wearing at the company where you are interviewing, and go for something similar, or a little nicer. I would say at least slacks and a button up shirt, but if the place is very casual (jeans and t-shirts) you don't necessarily need a tie. On the other hand, if you are looking to work at a law firm, your very best suit is the way to go.
Remember that the company wants to know what you can do for
them. Slant your answers to show that you are aware of this. Speak clearly in full sentences, try not to slouch, and look the interviewer in the eye.
Try to come up with examples of times that you have practiced the skills they are looking for. For instance, if you are looking at a customer service position, try, before-hand, to think about times when you have solved people's problems, or helped them out in some way. Employers are looking for someone who (preferably) has experience doing the kinds of things you will be doing, or at least who is smart enough to be able to figure things out quickly. Answering with examples gives you a chance to show that you have experience that will be helpful to them, and that you understand what they are looking for.
Also, try to think of the things a
manager will be concerned about on the job, before you go in. If there is cash handling or sensitive information involved, they'll want to know that you can be trusted; a foreman will want to know that you are aware of safety issues, etc. Be prepared with examples of how you have dealt with these things before.
Again, a lot of what you'll need to say depends on what kind of job you are interviewing for. If it's a basic, unskilled, entry level job, all they'll care about is that you are able to present yourself according to company standards, are smart enough to learn the job, easy-going enough to take orders without a fight, and that you won't offend people. (No B.O. or bad language.

) If it's a skilled job, obviously they want to know what skills and training you have that will benefit them.
If there are any weird glitches on your resume (like long strings of time out of work and/or school) be prepared with an honest explanation as to what was going on, (you don't have to go into too much detail) and why that is no longer going to be affecting you. (Training is expensive, so employers don't like to have employees coming and going any more than necessary.)
I hope some of this helps. If you want to PM me with more details about the type of work you are going into and what your experience is, I could possibly offer more advice.
Jamie