The size of steel is just as important for a new knifemaker as is the type. This is especially true if you do not have a belt grinder. Removing steel by hand can take a lot of time if you have excessive amounts to remove. My favorite size is 1 inch wide by 1/8 inch thick. The vast majority of knives that I make come from this size of stock. It is amazing how many different sizes and shapes of knives that you can make from it. Even many knives that need 1.25 inch wide steel can be made from 1 inch wide material with only slight changes that people never even notice.*
As for the type of steel, I prefer ATS-34, D2 and CPM-S30V. These are all easy to find and are air hardening steel. That means places like Texas Knifemakers Supply can heat treat them. Some places will not heat treat steel that require a type of quench bath (water, dry ice, oil, etc...). A2 would be a good choice but it can be a bit pricey and it is not very stain resistant. If you take good care of your blade, that is not an issue. I find that many (most) people are so used to stainless blades though that they will rust the knife if it isn't stain resistant.*
The finish of the steel, as shipped to you, is something to consider as well. I pay the extra fee for "precision ground". That means that they started with a piece of steel thicker than 1/8 of an inch and ground off the mill finish so that it is exactly 1/8 of an inch thick. More importantly though is that all the little pit holes have been ground out and the "mill scale" has been removed. To me this is well worth the price. D2 usually doesn't have too bad of a mill finish and neither does ATS-34 once you get past the dark outer surface. CPM-S30V though can have a terribly pitted surface. Out of the three steels that I prefer, I have only found D2 in precision ground.*
I do not recommend CPM-S30V for beginning knife makers since it is hard to sharpen, expensive to purchase, expensive to heat treat, hard to grind and is a real bugger to polish. However once the knife is completed, it makes for a really great blade that gets a fine (sharp) edge and holds it better than the other steels that I use.*
D2 tool steel is probably my over-all favorite. It is more expensive to purchase than ATS-34 but is the same price to heat treat. It holds an edge better than ATS-34 but not as good as CPM-S30V. Although most people honestly wouldn't be able to tell the difference between D2 and CPM-S30V. With 12% chrome D2 is technically not a stainless steel, but it is very stain resistant. It is commonly used as planer blades in sawmills. The draw backs to D2 are that it is harder to sharpen than ATS-34 and it doesn't get as fine of an edge.*
ATS-34 is what I would recommend for the first time knifemaker if not D2. It is inexpensive and makes for a great knife. It is easy to grind and sharpen. Polishing it is easy once you get past the dark mill finish. The dark outer finish on ATS-34 can be a real bear to remove by hand. Even though ATS-34 doesn't hold an edge as well as D2 or CPM-S30V, it is still at the top of the pack. It is a premium steel that most production knife companies charge extra for.
Heat treating can be done by Texas Knifemakers Supply for $5 a blade if it is under 10 inches long. They do charge more for exotic steels such as CPM-S30V which can be as much as $25 a blade. Typically though the shipping there and back costs me more than the heat treating.*