What should I know?

I guess a lot is up to what you like working with, materialwise. I've only used polymer clay a little, but I thought it was too difficult to work with once the clay was cured although it was sturdy and water resistant, so I quickly moved onto airdrying clay. It had the problem of easily being dented and not good with liquids so it requires a primer, but it is easy to sand and work with after it's dried so it's easy to get a smooth result. They both have pros and cons so maybe you should ask yourself what's most important.

I do have quite a few good tips up my sleeve (just ask away!), but my first and foremost tip is this: look for metal tools used for games like Warhammer. They are amazing for sculpting details. You can also find a lot of handmade tools if you just look around a bit, tools for nails for example, which makes life much easier. My second major tip is that the smaller a doll is, the more difficult it will be to cast (at least that's my experience) using channels and similar. Bigger dolls are easier to cast but require a lot more materials so they can be expensive to cast if you plan on doing it yourself.

 

My best tip would be: Just get started!
You will need to read a lot, research a lot, experiment a lot and you will end up with a few botched parts and mistakes along the way, but if you don’t actually start your project, you will never get it done. There is no way in the world to learn absolutely everything that you need to know before you even begin.
For now, don’t be blinded by the great goal. To have a doll cast is a wonderful thing, but if you have never sculpted anything yet, that will be quite some time into the future.
So start out with any materials that you like and can afford. Polymer clay, epoxy putty, paper clay, sculpting wax… Heck, even newspaper and PVA glue works. It is a matter of preference, really and you can’t find your favorite without some experimentation.

Decide on a size that you enjoy working with. Make a plan. Take a deep breath and dive right in.
Of course, keep researching as you go. Whenever you encounter a problem, then you know what specific questions to ask, what books to buy and what words to put into the search engine.

 

even though i'm doing 3d sculpting, I'd say learn your anatomy! proportions, muscle layout, joint movement and whatnot. especially if you're trying to make a realistic looking doll.