You can do this through the properties window. Right click the User table and select Modify. Now select the 'id' column and look at the properties window at the bottom of your screen.
In the properties you'll see an Identity Specification attribute with a '+' sign next to it. Click on the plus sign and where it says (Is Identity), change it from a 'No' to a 'Yes'.
You'll notice that the change also updates the Identity Increment and Identity Seed properties (both should be 1, most likely, for your purposes).
Hope this solves your problem,