Getting UniqueID

Im currently trying to finish up my db so i can start working on my site more since i cant move forward till i finish the db... After a user creates account they can start posting stuff on the site and i want to link the post with there uniqueID that is made automatically from the log in creation wizard. So like how would i get that in uniqueID from the table and place it into another. I tried looking at the stored procedures that is created with aspnet_regsql.exe but it didnt help me much. hope i provided enough detail.