In Platinum, you cannot avoid encountering it. It should be pretty self-explanatory but here is a link to a walkthrough of Platinum showing you just how to get there.
Marriland's Platinum Walkthrough (Giratina): 
I'm assuming you're talking about Diamond and Pearl, though, otherwise your question would be unnecessary. After you get the National Dex, go to Route 214. You will see a new path there that wasn't there before. Follow it to Turnback Cave. You will find Giratina here, at level 70.