The best questions to ask are those that you really would like to know the answer to, rather than those you can find in books on interview skills. If you research the company well enough, you will find a number of questions naturally arising that you wish to be answered.
You should, though, concentrate on questions that show your interest in, and motivation to do, the job itself, rather than the rewards it will bring. So, for example, you should ask about training and career progression in preference to pay and pensions!
Some examples of questions you could reasonably ask at interview:
"What are other recent training contract students now working on?"
"Are there any particular types of client which I might expect to work with when I first begin work?"
"Are there any key developments planned for the firm over the next few years?"
"How do you market the firm?"