A sprint planning session is held on the first day of a sprint.
A collaborative effort facilitated by the project manager. The project manager explains the backlog and defines the sprint goal. The development team then defines the work and effort needed to meet the goal.
Go through the backlog and familiarise yourself with the user stories.
If any story has a dependency on another system note that down.
Roughly estimate the effort needed to do a story.
Estimate each issue using function point estimate.
If you're unaware of the measurement criteria please reach out to the project manager.
Raise any blockers or concerns you have for an issue before estimation.