You need to write the Query this way.

DECLARE @ContactName VARCHAR(20)
SET @ContactName = 'maria'
SELECT customers.CustomerId
	,ContactName
	,OrderId 
FROM Customers customers  INNER JOIN Orders orders ON Orders.CustomerId = customers.CustomerId
WHERE ContactName LIKE @ContactName + '%'

Refer this link to download the Northwind database

Install Microsoft Northwind and Pubs Sample databases in SQL Server Management Studio

your C# query will be

string strQuery = "SELECT customers.CustomerId	,ContactName ,OrderId FROM Customers customers  INNER JOIN Orders orders ON Orders.CustomerId = customers.CustomerId WHERE ContactName LIKE @ContactName + '%'";

Use this link to know how to use parameterized query

Parameterized Queries ADO.Net