This looks like a homework assignment. If you're genuinely going to be working with this stuff, I suggest you look into how to write SQL - the basics aren't overly difficult.

To answer your question, to join those two tables you need to know which column in Customer indicates which Site each customer relates to, and then build a query like below:

SELECT CUSTOMER.CUSTOMER_NAME

, COUNT(SITE.SITE_ID) AS SITE_COUNT

FROMCUSTOMER

INNER JOIN SITE

ON CUSTOMER.SITE_ID = SITE.SITE_ID

GROUP BY CUSTOMER.CUSTOMER_NAME

HAVING COUNT(SITE.SITE_ID) >= 10