At some point i wrote out a example prism class for someone which is basically the same thing as a cone. If you flaten out the top of the prism you have a cone.
The full code is in the post linked below.
Specifically look to the methods BuildPrism(…) and GetPrismVertices(…) in the second part of it.
Basically all that example code can just be copy pasted to game1 if you want to run it.
Hope it helps.