The answer is not specific to HR and is applicable to all KPI for any team :
Why a KPI? -> to maximize the productivity/quality/delay/etc of the team to deliver on target.
How to definine a KPI -> first you need to identify what are the objectives of the team ( for HR : recruting whitout too much delay, contain the salary mass, organizee the office life, time to settle legal issues, etc).
Note : as you will need to explain to your boss how you set up the KPI, you can do a PPT slide with a frame : on the left all the input of the team ( order from CEO, request of hiring from other department, law of the country, etc) and on the right the output (how many people hired, turnover, no legal complain). You should have a KPI on each output ( and input if you want to justify why there in an issue an on output that is not caused your team)
How to set a KPI : for each KPI you need to set level of achievement with a value (low turnover is good, high turnover is bad) (1 month to find a candidate is good, 1 year in bad)
How to weight a KPI -> if you have 5 KPI you can weight them each 20% if you think they have the exact same importance. But usually each KPI will have a different weight. ( it is more importnat to follow the KPI of the recruting process timeframe than the KPI of cofee machine downtime - or is it? :) ).
Knowing that you can EDIT your post, propose your set of KPI, and we can review it.