What are you using for documentation and reminders for licensing and cert renewals?

I work on a small team that is all relatively new with the most senior person on the team being there 2.5 years and the rest less than 1 year. With everyone that built and managed the IT infrastructure retired or fired and the current documentation unorganized or incomplete and outdated this is the perfect opportunity to build documentation and learn the business.

What are some tips to build great documentation? What would you prioritize first?

What free or paid software can help with this goal?

Whats the best way to track licensing and cert and other recurring IT tasks?

I want to take the time to do this right to build the skills and truly help the rest of the IT team.