Hola Melissa 

We often use the preposition "de" for description, so, if in English you say "a plastic doll", in Spanish we say "una muñeca de plástico". Here's a lesson on this use of "de".

You can also make it more literal and say "made of plastic" by using "estar hecho / hecha de + [material]:

Esta una muñeca está hecha de plástico. 

This doll is made of plastic. 

Here's another lesson on this other specific topic. 

I hope they help.

Saludos cordiales

Inma