First things first, since you clearly missed it.
Forum Etiquette: How to post data/code on a forum to get the best help[/url]
If you post some consumable data, this probably isn't that hard. Something like a
ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY SomeDate)
would give you a sequential numbering... then you can just find the nth one or whatever (just filter for it in your WHERE clause).