Sql Server queries optimization techniques - stored-procedures

Question

My Stored Procedure takes a very long time to execute.
Can anyone suggest me what I can do to speed up the stored procedure, apart from using some good practices for writing down the queries.
I've heard about creating indices, but I'm not sure what are they.
Please suggest all the best ways to speed up my queries.
Thanks
CREATE PROCEDURE [dbo].[usp_GetAlternates]
(
#NNumber CHAR(11) ,
#pid INT ,
#pbmid INT
)
AS
BEGIN
TRUNCATE TABLE TempTherapeuticAlt
INSERT INTO TempTherapeuticAlt
SELECT NULL AS MedicationID ,
PR.ePrescribingName AS MedicationName ,
U.Strength AS MedicationStrength ,
FRM.FormName AS MedicationForm ,
PR.DEAClassificationID AS DEASchedule ,
NULL AS NDCNumber
FROM Product PR
JOIN ( SELECT MP.MarketedProductID
FROM table2 TCTSP
JOIN table3 MP ON MP.SpecificProductID = TCTSP.SpecificProductID
JOIN ( SELECT TCTSP.TherapeuticConceptTreeID
FROM table3 MP
JOIN table2 TCTSP ON MP.SpecificProductID = TCTSP.SpecificProductID
JOIN ( SELECT
PR.MarketedProductID
FROM
table4 PA
JOIN Product PR ON PA.ProductID = PR.ProductID
WHERE
PA.NDC11 = #NNumber
) PAPA ON MP.MarketedProductID = PAPA.MarketedProductID
) xxx ON TCTSP.TherapeuticConceptTreeID = xxx.TherapeuticConceptTreeID
) MPI ON PR.MarketedProductID = MPI.MarketedProductID
JOIN ( SELECT P.ProductID ,
O.Strength ,
O.Unit
FROM Product AS P
INNER JOIN table3 AS M ON P.MarketedProductID = M.MarketedProductID
INNER JOIN table5 AS S ON M.SpecificProductID = S.SpecificProductID
LEFT OUTER JOIN table6 AS O ON S.SpecificProductID = O.SpecificProductID
GROUP BY P.ProductID ,
O.Strength ,
O.Unit
) U ON PR.ProductID = U.ProductID
JOIN ( SELECT PA.ProductID ,
S.ScriptFormID ,
F.Code AS NCPDPScriptFormCode ,
S.FormName
FROM table4 AS PA
INNER JOIN table7 AS S ON PA.NCPDPScriptFormCode = S.NCPDPScriptFormCode
INNER JOIN table8 AS F ON S.FormName = F.FormName
GROUP BY PA.ProductID ,
S.ScriptFormID ,
F.Code ,
S.FormName
) FRM ON PR.ProductID = FRM.ProductID
GROUP BY PR.ePrescribingName ,
U.Strength ,
FRM.FormName ,
PR.DEAClassificationID
ORDER BY pr.ePrescribingName
SELECT LL.ProductID AS MedicationID ,
temp.MedicationName ,
temp.MedicationStrength ,
temp.MedicationForm ,
temp.DEASchedule ,
temp.NDCNumber ,
fs.[ReturnFormulary] AS FormularyStatus ,
copay.CopaTier ,
copay.FirstCopayTerm ,
copay.FlatCopayAmount ,
copay.PercentageCopay
FROM TempTherapeuticAlt temp
OUTER APPLY ( SELECT TOP 1
ProductID
FROM Product
WHERE ePrescribingName = temp.MedicationName
) AS LL
OUTER APPLY function1(#pid, LL.ProductID, #pbmid) AS fs
OUTER APPLY function2(LL.ProductID, #pbmid) AS copay
ORDER BY LL.ProductID
TRUNCATE TABLE TempTherapeuticAlt
END
GO

Answer 1

Here are a few:
You should have indexes for every column in a WHERE clause. See
your SQL language for how to do it.
Learn how to EXPLAIN PLAN and see what's slow.
Stored procedure languages are functional, not set based. Use JOIN and don't fall into the (n+1) query/iteration trap.
Understand how using certain functions force you to TABLE SCAN in a WHERE clause.