You're probablt best off to go one of either two ways:
1) Buy the albums in order (Orchid, Morningrise, My Arms Your Hearse, Still Life, Blackwater park, Deliverance, Damnation), or
2) Go accessibility-wise. Blackwater Park and Still Life are both pretty accessible, and showcase the spectrum of their styles (to an extent). These would probably be your best bet. Although, in the long run, you'll end up loving the first three albums: they're fantastic - but probably a touch harder to get into if you're just starting to get into Opeth.
Anyway, I'm sure you'll love whatever you pick up if you give it time. Enjoy!