Welcome to the fun. Yes, you ARE new - I remember when I didn't know the meaning of thinset, and it appeared no one would tell me. It's a cement-based mortar for applying tiles. If you are going to mosaic your concrete goose, that's what you should use. I use the Mapei brand from Lowes. It already has the polymer admixture in it. I'm sure Home Depot has mortar w/the polymer, but have never used theirs. It's found in the flooring section at Lowes. I buy it in 50-lb bags, but you can get it in smaller bags. Be sure and post pictures of your work in progress (WIP).

yep, go to your tile section of the hardware store and ask for thinset, comes in white or light grey.

I've yet to try this, but in a mosaic class I took we learned to mix up small amounts of thinset in a zip-lock baggie and cut the corner off after it's mixed. Then you use it like a frosting bag. Less mess, less waste.
Becky