The King Kam beach dive Chris mentions is easy, and a good dive, unless there's a big south swell.
It's hard to go wrong with the Place of Refuge. 150' to the water and you are in diveable depth right off the rocks. There are likely to be lots of divers as this is probably one of the best shore dives in the state. I wonder if the earlier poster missed the 2 steps entry, there are few easier entries and exits in Kona unless he did it on a nasty day vs. a good day at other spots. Another disagreement, Puako, while a great dive, might be a tough entry for newly certified divers without a guide- most of the entries require a 50 yard or more swim/wade across shallow water to the drop off, any waves will seem magnified (I've seen experienced shore divers have their butts kicked in 18" surf here) - once you get some experience, then persue this dive.
Mahukona up north is an easy dive, easy entry, but it's a long ways and probably not as nice as some of the closer stuff.
Google Dick Dresie and his "Let's go shore divin'" website for some pointers and site descriptions.
Aloha,