Probably the best varieties are old favorites Chandler, Gaviota or Seascape -- taste and see which seems better from that farmer on that day. The main commercial variety is Camarosa, which can be a good berry if it is harvested when it is nearly black.
Choosing good ones is pretty simple: your nose will know. Sniff around until you find a stand where the berries smell so good you can't resist. That's all there is to it, though you should also check the bottom of the basket to make sure the berries haven't gone over the hill and started leaking.
You can refrigerate berries, but I think the flavor is best if you leave them at room temperature and eat them the same day. Wash gently in cool running water, pat dry and then hull them, removing the green top (removing it before washing will cause the berries to absorb more water).