Using DetailsView to add User ID?

Hey guys I'm pretty new with all this stuff and I'm stuck! Basically I'm using the Sqlserver aspnet_regsql.exe for my database. I have added my own tables to this database, these tables are just for items. In one of my items tables I have an "user_id" row so I can tell which user posts an item. So I was just wondering, when a logged in user uses my details view to post an item, how can I automatically add their user id to the table? I am using VB, not C#. I'm not sure which code to use to get their user ID and store it along with the other details they type in.

thankkkksssss