Seeking advice from supervisors

Hello everyone! Upcoming trainee here, and am quite anxious about my TC. To those with experience supervising juniors, I’d be grateful if you could share some words of advice and experience re the following three questions:

- If a supervisor assigns new work and I do not have capacity to take it on, how should I convey this? Essentially, how do I reject a piece of work?

- If I were given a task that, after research, do not know the answer to, is the formula to set out: what you’ve done, and then specific qs relating to the task?

- How do you strike the balance of updating your supervisor re the progress of your task, vs not being overly bombarding?

Thank you in advance!