Congratulations on your decision!! I've been Oil Pulling for three months now and have absolutely no plans to quit. Personally, I think that you ought to try out the lighter oils (Sunflower, Safflower) before you give the heavier oils (olive and hemp) a shot. However, if all you've got is Olive Oil in your pantry, by all means, start with that. :)
Just keep in mind that the heavier oils won't generally turn white and you'll be more likely to get a "healing crisis" right away as opposed to easing into the therapy and allowing your body to slowly work up to the toxin release.
Over all, my advice is, don't worry about a thing and start pulling that oil!