Replace "modID" with your actual mod ID and include your mod ID in the unlocalised name (e.g. by making it the String form of the registry name ResourceLocation).
The numeric ID can be any number unique to your mod, though it's probably easiest if you start from 0 and increment by 1 each time you register an entity. It's used in the entity spawn packet.
You can see how I register my mod's entities here.