This is all new to me, (sql) and now I am trying to read up on how to make Selects more efficent and faster.

This is one of my main statements that takes most of the time (68%)

Code:
SELECT     CONVERT(varchar(4),HDR.MarketingClockNumber), HDR.PlantNumber, HDR.QuoteNumber, HDR.EffectiveStartDate, HDR.CustomerNumber, 
                      	HDR.CustomerName, (ITM.NetUnitPrice + SUM(ISNULL(ITX.Adder, 0))) * ITM.Quantity * ITM.DiscountMultiplier AS QuoteNetValue, 
		HDR.YourInquiry, ITM.LineItem  
FROM         DataWarehouse.dbo.QuoteHDR HDR  LEFT OUTER JOIN DataWarehouse.dbo.QuoteITM ITM ON HDR.PlantNumber = ITM.PlantNumber AND HDR.QuoteNumber = ITM.QuoteNumber LEFT OUTER JOIN
                     		DataWarehouse.dbo.QuoteITX ITX ON HDR.PlantNumber = ITX.PlantNumber AND HDR.QuoteNumber = ITX.QuoteNumber AND ITM.LineItem = ITX.ItemNumber
	WHERE     (HDR.PlantNumber = '12' OR
                      HDR.PlantNumber = '18' OR
                      HDR.PlantNumber = '19' OR
                      HDR.PlantNumber = '22' OR
                      HDR.PlantNumber = '23') AND    (ITX.PlantNumber = '12' OR
                      ITX.PlantNumber = '18' OR
                      ITX.PlantNumber = '19' OR
                      ITX.PlantNumber = '22' OR
                      ITX.PlantNumber = '23') AND   (ITX.PlantNumber = '12' OR
                      ITM.PlantNumber = '18' OR
                      ITM.PlantNumber = '19' OR
                      ITM.PlantNumber = '22' OR
                      ITM.PlantNumber = '23') AND (HDR.EffectiveStartDate BETWEEN CONVERT(DATETIME, '2004-10-25 00:00:00', 102) 
		AND CONVERT(DATETIME, '2005-6-30 00:00:00', 102)) 
		AND (HDR.QuoteNumber NOT IN
	                          (SELECT QuoteNumber  FROM QuoteFeedback.dbo.QuoteTracking   ) )  GROUP BY HDR.MarketingClockNumber, HDR.PlantNumber, HDR.QuoteNumber, HDR.EffectiveStartDate, HDR.CustomerNumber, 
		                      HDR.CustomerName, HDR.YourInquiry, ITM.DiscountMultiplier, ITM.Quantity, ITM.LineItem, ITM.NetUnitPrice
For indexes, i have clustered, Plant Number, QuoteNumber, and LineItem. Here is the Execution Plan.

QuoteHDR has 0.2 Million
QuoteITM has 0.5 Million
QuoteITX has 1.5 Million