how to create paramterised sql function?

Question

I have one table, in that i have many column name.

but in function there is use of only three column 

 

I want to calculate and insert data in column NO_OF_MONTHS from START_DT to todays date. and there is one more column end date

 

function should have paramter @start_dt and end_dt

 

suppose start date is 1-1-2018 and end_dt is 1-1-2020 

the  function should insert 12 in column NO_OF_MONTHS because its 12 months passed from 1-1-2018.

 

I hope you guys understand my question.

Thanks waiting for reply.