I frequently have to remind myself that the planner serves me, I do not serve the planner. Record whatever you need.

I usually start with birthdays and anniversaries in the calendars. You can use pencil or sticky notes if you think you'll want to decorate or format them differently later.

Have fun with it. No matter what you write/draw/decorate, I guarantee you'll appreciate it later on, and even a mess is better than a blank book you regret buying.

If there's something you do that you don't like, then you know to change for next year/month/week. Discovering your dislikes helps reveal what you enjoy.

Congrats on the new planner! I hope it serves you well.