How to automatically assign an ID number

How to automatically assign an ID number

I'm creating a property database and I need to assign an ID to each property acquired. I'd like the ID number to be automatically assigned so that users don't have to input an ID and also so that they can't adjust the value. I'd like the first property to be assigned an ID of 100, then the next 101, then 102, and so on. Does anybody know how I can do this? Thanks for your help.