Thank you! I hope you don’t mind that I respond to this publicly, but I’ve been asked this a couple of time recently.
First, I don’t claim to be an expert of this stuff. I am still very much learning and experimenting!
Second, a little bit of background on me. I started actively trying to learn to do special effects stuff when I was in college around my sophomore year, so around 1998. Keep in mind that while the internet was around then, there still wasn’t a whole lot of information out there. Also, I didn’t have a computer, so I had to go to the library to do my research. So I read what I could and experimented on the rest.
At that time I was working at the college art supply store, so I got the materials that I could get there since I got them at a discount. I remember starting with Van Aken Plasticine clay for the sculpt and then plaster bandages for the mold and latex used for model railroads for the positive. While not optimal or recommended, I actually got a few nice pieces. I even won an award with a photograph I took of my feet with extra toes glued on (it may even still be displayed at UW-Madison :P).
Nowadays there is so much information out there, so I recommend reading anything you can get your hands on (I still do this).
Monster Makers http://www.monstermakers.com/ is a great place for understanding the basics of mask-making. They have a few starter kits, which I’ve only heard good things about. They are focused on Latex mask making, but a lot of the advice transfers over to different mediums.
Smooth On http://www.smooth-on.com is filled with tutorials and advice on how to use resin and silicone. I regularly use them as a resource.
The RPF http://www.therpf.com/ is a great forum filled with lots of industry professionals willing to share their wisdom. The sculpture/effects board is particularly useful.
As far as starter materials, I recommend getting your hands on some clay and sculpting tools and just go for it. I prefer oil-based clays like Plastilina or Monster Clay (just make sure you get non-sulpur clays if you’re planning on molding it). Others prefer WED clay. You can read about both on the Monster Makers website.
You should be able to find a basic tool kit at any craft/art supply store. After that you can add to it with tools that you think will fill a niche for you, or even make your own tools.
Mold-making is a lot more complicated than just starting sculpting, and I recommend you thoroughly research whichever method you decide to go with. Each material has different requirements, don’t rush into it! Your first molds will probably be pretty bad because it is a steep learning curve. Don’t let that discourage you! With practice and work you will get better!
tl;dr- read/observe/learn from whatever you can and practice makes you better.