A: Read the site. This question usually comes from kids who want to make their favorite super hero costume or want a lightsaber but they haven't bothered to read up on the topic. Everyone who is -actually- serious about making props and costumes will research, do more research, and then research some more. If you browse through prop message boards at all, you will see many people with initial posts just like the one up there. They are expecting to have their hands held through the process. And when they don't get a response, you never see them again.
As much as you would like to, you aren't going to learn everything all at once, and a lot of the learning comes as you go. You will make mistakes, you will waste a ton of money, and screw up big projects. You learn from it. And making mistakes like that is a reminder to work within your skill level. Gradually build it up. If you have never done a bit of prop work, chances are this beautiful project you have in your mind won't turn out anywhere near as cool as you imagine.
My advice is just start with something easy. Learn how to make lightsabers out of plumbing parts, learn how to modify existing toys to make them look like cool sci fi weapons, learn how to repaint toys, etc etc. Once you get used to making props out of existing items, then try your hand at making custom pieces.
When you start to learn how to use the supplies, do small scale projects. When you are still learning how to work with silicon, make a mold of something small such as a Power Rangers Morpher Coin. Very simple and straight forward thing to mold. Same thing when working with the resin/plastic for the first time. Small scale.
So there, a very long answer to one of the most frequent, unnecessary, and annoying questions one can ask.
Q: This stuff is expensive, is there anything else I can use?
A: Another variation of this question is "I'm on a budget. Can I make -insert prop here- for -insert miniscule amount of money here-?" It has been my experience that 'craft store' supplies are pretty horrible. You can buy resin and molding supplies there(usually clear resin and latex mold builder), but really the only things you want to get from a craft store are clay and paint. There is a reason people use the expensive supplies, and that is because they work, and work very well. This is an expensive hobby. Save your money, spend it wisely, and use your supplies wisely as well. They can last a long time.
Q: What supplies do you use? What supplies should I use?
A: A wide variety. I go into more details on my "Supplies" page in the Tutorials section, so check that out. I use whatever the project calls for. You will eventually develop you own preferences as far as what company and which of their supplies you like. So far I have had a lot of success with a company called Smooth-On. They sell the molding silicon and the liquid plastic/resin.