Learn some general coding technical skills for the technical interview and do some background research on the company.
Go through the common technical interview questions for the technical interview, and be confident and considerate in behavioural and group interviews.
Personal and group assessment centres are very hard to prepare for and I feel it shouldn't be prepared for at all and should be done based on your true personality and feelings. As such, the full focus should be prepared on the technical component of the interview, such as knowing about networks/databases/concurrency/algorithms/etc.
Prepare for basic IT questions, algorithms and data structures, be yourself - that should be enough
Make sure in the assessment centre you put yourself out there with the Lego building - show off your team working skills.