Function To Calculate Holidays

Hello,

First things first - I have a table which holds employee information (tbl_EmployeeDetails), and another table which holds information about the holidays they have booked (tbl_Holidays).

If an employee books 5 days off, 5 rows will appear in the tbl_Holidays table, each line showing 1 under the day field and 7 under the hours field.

I'm trying to produce a function which will do the following :

1) Pass in the employee number
2) Establish whether the employee works full time or part time by looking up to tbl_employeedetails, and checking the fulltime flag
3) If full time, look up to tbl_Holidays and count number of days
4) If part time, look up to tbl_Holidays and count number of hours
5) After this, return the number of holidays booked

My code is as follows :
=============
CREATE FUNCTION [dbo].[fn_Get_Booked_Holidays_Current_Year] ( @EmpNo int )

RETURNS Float
AS
BEGIN

-- Declare fields
DECLARE @FullTime int

-- Determine if Part Time or Full Time
SET @FullTime = SELECT FullTime FROM tbl_EmployeeDetails

IF @FullTime = 1
SELECT COUNT(NumberOfDays) AS TotalHolidays, EmployeeNumber AS EERef
FROM dbo.tbl_Holidays
GROUP BY EmployeeNumber
HAVING (EmployeeNumber = @EmpNo)

IF @FullTime = 0
SELECT COUNT(NumberOfDays) AS TotalHolidays, EmployeeNumber AS EERef
FROM dbo.tbl_Holidays
GROUP BY EmployeeNumber
HAVING (EmployeeNumber = @EmpNo)

END
==========

Can someone please let me know where I'm going wrong, or what I need to do to get this function done?

Thanks,
J