Buying first cast iron pan size/depth/shape
I'd offer the following recommendations if someone hasn't worked with cast iron before:


Cast iron is heavy.  If you're not used to them, and used to flipping things in your pan by just lifting and shaking ... it will not go well until you've gotten used to it.  You'd likely be better off with a smaller pan, provided that it's of sufficient size for the amount you're cooking.  If you're only cooking for 1 or 2 people, a 10" (25cm) pan is likely better than a 12" (30cm) one.
If you're going to be grilling things, but not making sauces or sautéing, go with a griddle.  Round griddles heat up much more evenly (less issues w/ cold spots near the walls), are easier to clean, and are significantly lighter than cast iron pans.  Grill pans are really only needed for things that are going to drip significant amounts of fat, or if you really like grill lines.
If you buy a larger pan (14"/ 36cm) or larger, make sure it has a second handle.  I've seen some that just have a little nub and not much else, which makes it very difficult to lift when it's loaded down with food, as you don't always have good grip with the potholders.
Pour spouts are nice to have, as you don't want to hold the pan for a long time trying to be careful when pouring.  (heavy pan + liquid weight is not a good combination).  The only disadvantage to the spouts are that you can't lid it as tightly, which may be an issue for pots, but is generally not a problem for pans.
Learn how to hold the pan when transfering food out of it.  You'll notice in some cooking shows that the chefs use a strange grip : with your non-dominant hand, reach across the pan and grab the handle so that your palm is under the handle.  Grip tightly and lift over your serving dish, and then scrape things out with your dominant hand.  You will tire out much more quickly if you try to hold it with the handle to the side.
You only need a high-sided pan if you're going to be really loading up the pan (eg, trying to wilt down a bunch of greens), braising, or shallow frying.  (it's most important when frying).  Higher sides add extra weight, and less even heating.  
Figure out where you're going to store it.  As they're heavy, you likely don't want to be storing them up high (hazardous) or down low (back problems) if you can avoid it.  You also can't stack them all on top of each other on a flimsy shelf.  I wouldn't hang them from a pot-rack if it was overhead.  (maybe on a wall-mounted one, where I knew it had good lags into studs)  If you just have one, you can clean it and leave it on your stovetop ... once you start getting a few different sizes, you have to figure out where to put them.
Invest in a handle cover.  You can get ones that are padded cloth, leather, or silicone that slip over the handle, and just stay on while you're cooking.  You don't want pans where the handle is a separate part from the cast iron.
Enameled cast iron is easier to clean, but it will start to stain and craze after years of use.  I like enamed for pots, but generally prefer seasoned for pans.
Make sure to read the manufacturer's directions for cleaning and seasoning your pan before the first use.  Seasoned pans may come 'pre-seasoned', and just need a touch up before you start using it, but bare metal pans generally need to be stripped down of whatever protective coating they shipped with, and then seasoned, which can take a couple of hours to build up some layers.  (it's easier to do in the oven, but slower ... I still recommend the oven so you can make sure to season the underside and handles of the pan, which frequently get forgotten)


... and whatever you do, do not start collecting cast iron because it's decorative.  I have less than a dozen, and 3 of them get heavy use (14" skillet, 12" round griddle, 7qt enameled pot).  I had to work up to the 14" skillet ... starting out, it was a 10" until I got used to working with it, which is still a great size for cornbread and such.  ... but my mom has well over 100 pieces, because she's collecting Griswold and Wagner.  (and she's got at lot of it on an antique wooden baking rack that I'm amazed hasn't broken / collapsed considering that it's significantly sagging)