Gemmy, can I ask how you study for a behavioral exam? Just go in there willing to be open an honest and answer the questions as best you can.
You will be asked to recall events in your life based on an example. For instance, you may be asked to tell them about a time where workplace safety was in jeopardy. You will be asked what the situation was, how you were involved, what you did and what the outcome was.
Don't just say 'one time there was some stuff lying on the floor so I put it away' - that will not go well. Be as specific as you can - dates, people involved, what you said, what you did, what outcome there was. That's what they will want to hear.
The only study you can do is to think back to times in the workplace (or school, or home, or whatever) where situations have arisen where you have been involved and there was an outcome - good, bad or otherwise. I am certain you will get some questioning about conflicts in the workplace - have a think about times where you have been involved or have witnessed conflicts at work.
Hope it goes well for you. ATC can be a very rewarding and exciting profession. Make the most of this opportunity and give it your best shot!
Cheers,
NFR.