INSERT INTO Customers (FName, LName, PhoneNumber) SELECT FName, LName, PhoneNumber FROM Employees
This example will insert all Employees into the Customers table. Since the two tables have different fields and you don't want to move all the fields over, you need to set which fields to insert into and which fields to select. The correlating field names don't need to be called the same thing, but then need to be the same data type. This example is assuming that the Id field has an Identity Specification set and will auto increment.
If you have two tables that have exactly the same field names and just want to move all the records over you can use:
INSERT INTO Table1 SELECT * FROM Table2