I am trying to use a small silicone baby mold, what material do I use so that it holds it's shape?

I hope that I am reading you question correctly but you need to use either gumpaste or fondant or a mixture of both.

When is your project due? What are you trying to mold?
-chocolate
-gumpaste
-fonadant
-poured sugar

All of the above will harden w/in various times.
Each will give you a differnt effect on your project. It also is important as to what you have on hand & how much time you have as to which you can work with. Obvously gumpaste & fondant will work well but take time to dry. If you want quicker results you can use poured sugar in those molds too & they will dry reasonably quick... but humidity will take it's toll.

I would use merckens white chocolate..
- Melt the chips in the microwave 30 secs at a time, stirring after each period is up... (decreasing the amount of seconds by 5 and re-nuking it)..
- Once it's ready (liquid) pour it into your mold.
- & then take the flat edge of a knife & level it off (putting the un-needed chocolate back in your bowl.
- Place the mold in the fridge for about 10-15 mins. & it will be ready!
- NOW... it is important to handle carefully as the heat of your hands will melt it.
- Place it of a plate dust it w/edible color & you are redy to put it on your cake!!

Thank you so much for the help. I am making a baby for a cake topper for a baby shower. I have never used a mold before and was not sure if the fondant would hold up. Thank you again for the help.

TayNat - You can use fondant in your baby mold. I like to dust the mold with cornstarch. Don't use powdered sugar because sugar can get sticky when in contact with fondant. Place cornstarch in the foot cut from a pair of tights. This is called a pounce. Tap your mold with the pounce and cornstarch should cover the entire inside. Smack the mold upside down so excess cornstarch comes out. You should have a complete and perfect, light dusting of cornstarch on the inside of your mold. Now press your fondant in with small pieces first to capture the details and then fill the mold with fondant. You can try to unmold, but if you want you can put the filled mold in the freezer for about 10 minutes. The fondant will firm up but the silicone will remain pliable. This will help in getting the fondant out without distortion of the design.

Hope This Helps,

Dominic