Be able to discuss why you want to be a CNA, and why you are interested in the interviewer's facility in particular (do some research, at least look at the facility's website if they have one). You may be asked about a time you went above and beyond for a resident/patient (perhaps you have a story from clinical you could share), and a time when you had a conflict with someone and how you handled it. If it's not clear what shift(s) you are interviewing for, be ready for questions about availability. If you are asked a question that you don't have a prepared answer for, it's OK to take a moment and think of what you want to say. . .don't be too prompt with the response that you'd regret!
Be sure to dress professionally - a simple blouse and slacks or skirt with modest shoes will suffice - you don't have to look like you could be a cast member in "The Devil Wears Prada." Have a resume and references prepared.
Hard to say how soon after the interview you might get an offer - could be on the spot, could be after you follow up with the facility a few times, depending on how organized they are! Good luck!