Expect the usual set of "devils advocate" style questions.
You will likely be asked for information on competitors/competitive options (and don't say "none", there is always SOME alternative).
You will likely be asked what makes you a domain expert on the product/service you are proposing. How you expect to scale, what does it take to be profitable, how will the company grow, and so on.
Also, rumor has it that YC is very big on demos, you should be prepared to demo your concept in some way. Ideally on at last a bare-bones implementation, but if not on something more real than simply "slide-ware".
The interviewers may also want some background info on you. How you got "here", cool/significant things you have done, what makes YOU the person to invest in (remember, they are investing in You/the team far more than just the idea).
There is also many helpful blog posts online from other interviewees.
Your question is a little unclear. I'm assuming you're asking about the questions the YC folks will ask during the interview phase. I think the best way to prepare for that is to watch Paul Graham in action with other startups.
Paul Graham and Harj Taggar recently held a public session of their famous Office Hours at TechCrunch Disrupt. This is the second time they've done this. The goal was to reproduce the sessions they hold with the startups who participate in Y Combinator. PG spoke with six startup founders to flesh out their ideas, ask probing questions as he tried to figure out what they were setting out to do, and what they might need to change.
You should watch these two videos (video 1 , video 2 ). It's interesting to see how quickly PG can think through problems, and figure out what are the right questions to ask. He's not looking to stump you. He's just trying to determine how much potential your startup has.
In addition to watching these videos, make sure you have a demo ready. Also, look to the YC application as a guide. My experience is that they will revisit some of the questions on the application.
Keep in mind that there's a reason why you get asked those questions. No one is trying to stump you. There is no right way to answer question X. The answers to the questions are important for you to be able to articulate if you are going to be the founder of a company. You need to be confident and build their trust that you passionate about your customer and understand the market. If you don't know then say so, nothing turns away and investor quicker than the BS meter going off.