Plenty of CBAT help if you search this site or just Google it. I don't know how useful they are as I've never used them. When I joined every applicant did computer-based tests, no opportunity to practice, and I passed everything. However, remember it's about aptitude, ie inate ability; some people have it, some don't and practice doesn't always make perfect.
Again, plenty of info available about interviews if you look, though TSR doesn't allow information that is too specific so as not to give an unfair advantage. I will say use what you put on your application and highlight your leadership potential. There are set questions asked; answer them, don't deviate too much and be honest.