issues about positioning declare statement

[Anshul Sharma]

DECLARE @start_date DATE='2020-01-22'
DECLARE @end_date DATE='2021-06-13'
SELECT
(DATEDIFF(DAY, @start_date, @end_date) / 30.4375) AS number_of_months;

This is my query but error shows up regarding DECLARE position which is not valid, what steps should I take to correct this issue