There are several ways you can accomplish this.
One commonly used way is to use the Session object, which you might use (after a successful login) like:
[csharp]// where sUsername is a string holding the username
// where nUserId is an int holding the user's ID number
Session.Add("username", sUsername);
Session.Add("userid", nUserId);[/csharp]
It's up to you what you store off for user info. Session can hold objects too so if you have a user object you could store the entire user object off if you wanted to. You can get values out of session just as easily:
[csharp]string sUsername = Session["username"];
int nUserId = Session.Contents["userid"]; // variation[/csharp]
Sessions can expire so you'll usually want to check for nulls or wrap a try/catch around the code that grabs the values. In a robust app you might have the user re-login if their session expire.
Another similar way to track user info is with cookies. Cookies are a little more flexible. Cookies are always stored on the client machine (Sessions can be setup for different storage mechanics, but it's often server-side storage).
A simple cookie example:
[csharp]HttpCookie oCookie = new HttpCookie("logininfo");
oCookie["username"] = sUsername;
oCookie["userid"] = nUserId;
oCookie.Expires = DateTime.Now.AddHours(2);
Response.Cookies.Add(oCookie);[/csharp]
And to retreive values:
[csharp]HttpCookie oCookie = Request.Cookies["logininfo"];
string sUsername = oCookie["username"];
int nUserId = int.Parse(oCookie["userid"]);[/csharp]
And again - you usually want to check for null or use a try/catch because cookies can expire too.
The syntax would be similar if you're using VB.Net.
For more info on either, refer to MSDN, google, or search around here.
Paul