There are lots of HR resources out there and I’m pretty sure a lot of companies offer some HR training and online handbook resources. Maybe ask your company for their suggestion? Or, if you’re unsure, I used a company called AllMyHr.
Generally though, your handbook needs to include the policies your company wants employees to follow, any expectations the business has of the employee, and what your employees can expect from your business. Things like procedures, holiday requests, sick pay, maternity/paternity leave, dress code, etc all should be outlined in the employee handbook.
Other things like the company’s mission, goals, expectations, history, etc all should be included too. The handbook should also be updated regularly, at least once a year as far as I'm aware.