SQL - Dynamically build where clause

I wrote inline SQL in a C# .NET application but it is timing out so I am creating a SPROC in sql server.  

I need to pass in a parameter for the purpose of dynamically determining the where clause.  Any idea how to do this?

As an example - lets say I am passing in CheckDate as BIT, dtStart and dtEnd.

If CheckDate = 0 then

    Where MyDate > '1/1/13'

if CheckDate = 1 and the dtStart = '5/1/13' and dtEnd IS NULL

    Where MyDate >= '5/1/13'

if CheckDate = 1 and the dtStart = '5/1/13' and dtEnd = '5/31/13'

    Where MyDate >= '5/1/13' and MyDate <= '6/1/13'

Thanks