see..u can write user defined function in many ways.

  here i am explaing u one sample example.
  
   Public Function Func_Login(strUser,strPassWord)
          
       Browser().Page().WebEdit("Name:=UserID").set strUser
       Browser().Page().WebEdit("Name:=Password").setSecure 
crypt.encrypt(strPassword)
       Browser().Page().WebButton("Name:=Log In").Click

   End Function

 When ever u want to login to application u can call this 
function with parameter values for User and Password.

Example:
   sUser = DataTable.Value("UserName")
   sPassword = DataTable.value("Password")

' Call the Login function for loggin in to application.. 

 Func_Login sUser,sPassword

Note : the function can not contain Object repository. So 
before calling or executing this function u have to 
associate Object repostiroy to the Current Action where 
this function is called. And also should assoicate the 
function library file where this function is stored.

  If any queries..

    nanda.dreddy@gmail.com

Its better to write user defined functions by using 
Descriptive programing in notepad and save them as .vbs 
extension. we can upload those files in QTP and we can use 
those functions.