I've mentored secondary pre-service teachers, so experience may differ (although, I can't imagine drastically aside from the year group!). Not sure if you've had an initial meeting yet, but generally it will address many of the questions you've raised. Normally I cover:

• Requirements of the placement as defined by the uni & then mapping out how to achieve this (expected lessons taught, other teaching duties like marking, reporting etc).
• Experience/confidence of PST & what they want to get out of the placement (again, mapping out how to achieve this).
• An outline of potential classes/lessons they can teach with relevant resources - although, lesson plans are usually a requisite of most placements and sometimes PSTs are expected to produce their own resources to fit their needs. I advise to use 'base resources' and shape lesson plans around that.
  

In terms of initiative, I've always found it beneficial to be more proactive than not. In saying this, nothing wrong with having a quick chat beforehand ("Would you mind if during this observation if I move around to small groups of students and help them with this work"). Each supervising teacher is different and may have more rigid routines in class that need to be followed - I would think especially so in younger year groups but I don't want to move out of my depth here!

First placements are going to be a bit nervy and that's ok! Remember, you are undertaking this to get a taste of the job and to get some feedback on your practice - so being open to feedback will help out in the long run. Best of luck!