creating a stored procedure using a function

I have a table with the the following columns
CustID      CustName      PrincipleAmount      NumberOfYears      DepositType

I have created a function that sets the principle rate
CREATE  FUNCTION  [dbo].[IntRate](@NumberofYears decimal(7,4))
      RETURNS decimal(7,4)
AS
BEGIN
      DECLARE @rate decimal(7,4)
      if @NumberofYears>=20 set @rate ='.15'
      else if @NumberofYears >= 11 set @rate ='.125'
      else if @NumberofYears>= 6 set @rate ='.10'
      else if @NumberofYears>= 4 set @rate ='.075'
      else if @NumberofYears>= 1 set @rate ='.05'
      else set @NumberofYears = '0'
      RETURN @rate
END


no i need to be able to calculate the final amount taking variable from both of these tables
PROBLEM:
I am unclear how to call the variable from the function
problem trying to solve:
Final Amount (A) – Takes three inputs P, N, and Deposit Type and returns the final amount using either simple interest formula or compound interest formula, depending upon the deposit type. For the calculation of R it uses the rate of interest function.

formulas: A=PNR (for simple interest)
                A=P(1+r)^N (for compound interest)
so i know i need an if statement based on the deposittype


I need to know how to get started or how to call the rate from the function.
Thanks