I'd suggest you watch The Vampire Diaries first if you haven't at least up to the end of last season then you can pretty much watch the two shows this season concurrently. You could probably watch just The Originals but you'd appreciate their back story more if you watched the other one first.
Agreed, probably better if you watch The Vampire Diaries first. You have no attachment to the characters and don't know their backstories otherwise. Definitely don't watch The Originals first if you intend to watch TVD though, it would ruin much of the suspense of a certain season. And after you watch The Originals you will likely want more, so it's probably just better to watch it in chronological release order in the first place. =]
Shows I'm Watching: Big Bang Theory, Glee, The Walking Dead, The Vampire Diaries, The Originals, How I Met Your Mother, Once Upon a Time, New Girl, Modern Family, Revolution, and a bunch of stuff like Pawn Stars/Storage Wars/etc. Also waiting on Grimm to start back up on Oct 25th. Not sure about any of the other new shows this season, will have to check some out I guess. Any recommendations?