Indexes

I've created indexes for the queries below running select getdate() before and after the query to determine the time. I'm curioius as to how sql server determines and decides how and when to use the indexes for the queries and how the run times of these queries compare. I'm also curious to know what kind of difference there would be on the last two queries as the only difference is the values.
For the last two queries I don't think there would be a difference but hoping that someone would know.

Below are my results:

select count(distinct CustomerID)
from Orders
where Status = 5

SELECT getdate()
go
Create Index Orders_Index On Orders(customerID)
go
SELECT getdate()


select sum(TotalDue)
from Orders
where CustomerID = 11212

select getdate()
go
Create Index Orders_Index On Orders(customerID)
go
select getdate()


select count(distinct AccountNumber)
from Orders
where SalesPersonID = 288

select getdate()
go
Create Index Orders_Index On Orders(salespersonID)
go
select getdate()


select count(distinct AccountNumber)
from Orders
where SalesPersonID = 276

select getdate()
go
Create Index Orders_Index On Orders(salespersonID)
go
select getdate()