How to add multiple atom types

I want to have two types of granular atoms, one with properties of glass and the other a ceramic, how do I do that? I know that the Youngs modulus etc has to be set with the fix command with as many values as there are atom types. But how do I declare there to be two atom types?

Sorry for the basic nature of my question - I know DEM but still grappling with LIGGGHTS commands and fluency.