Need Help Writing Stored Procedure Involving Dates

I am trying to write a stored procedure which would execute following logic:

- The stored procedure takes 2 optional parameters @StartDate and @EndDate

@StartDate Datetime = null
@EndDate Datetime = null

- Since the parameters are optional user can enter either one or can leave both blank.
- If user doesnot enter any values for SD (start date) and ED (end date), stored procedure should run select query replacing those values with wildcard character '%' or NULL
- If user enters SD, query should use @StartDate as the SD and GetDate() as the ED
- If user enters ED, query should use @EndDate as the ED and MIN() of the Date field as SD

I was able to write query which did almost everything as is stated above expect for incorporating NULLs
The query is as below

CREATE PROCEDURE SearchDocumentTable

@FName varchar(100) = null,
@LName varchar(25) = null,
@ID varchar(9) = null,
@StartDate Datetime = null,
@EndDate Datetime = null


AS
IF ( @StartDate IS NULL)
Select @StartDate = MIN(DateInputted) from Document

Select
FName as 'First Name',
LName as 'Last Name',
ID as 'Student ID',
Orphan as 'Orphan',
DocumentType as 'Document Type',
DocDesc as 'Description of the Document',
DateInputted as 'Date Entered',
InputtedBy as 'Entered by'

From Document,DocumentTypeCodes
Where FName LIKE ISNULL(@FName,'%')
AND LName LIKE ISNULL(@LName,'%' + NULL)
AND ID LIKE ISNULL(@ID,'%' + NULL)
AND (DateInputted BETWEEN @StartDate AND ISNULL(@EndDate,GETDATE()) OR DateInputted IS NULL)
AND Document.DocTypeCode = DocumentTypeCodes.DocTypeCode

GO

Any help would be appreciated
Thanks in advance :)