SQL IF inside a Function

ALTER FUNCTION [dbo].[FSmapping] (
    @runID as bigInt,
    @TypMap as char(1)
    )

RETURNS TABLE
AS
RETURN


IF (@TypMap ='C')
BEGIN
SELECT b.[A/C],g.accountNumber, b.[Process], g.[endingBalanceLC], g.beginningBalanceLC
FROM table1 g
LEFT JOIN  table2  b ON b.[Acc2]=g.Account_Synt
where RunID = @runID
END

I am trying to implement a table function in SQL. Basically it should have 3 more IFs and the only thing that's going to switch is the table2, from table2-4 based on inputs C I R S. The error is the IF statement, I know that something is missing, I am having a hard time implementing some sort of a switch function with table as a return value. The Select works flawlessly without the IF syntax, it also says errors on the two variables inside the IF

Must declare the scalar variable runID and TypMap