First create a user class something like below,
public class UserProfile
{
public string UserName {get;set;};
.........................
}
then on successful login,just create one static user object like ,
static UserProfile userProfile=new UserProfile(); and assign the username
userProfile.UserName="XYZ";
then you can access that object in any form in your application(in your case on next form).
lblUsername.Text=UserProfile.UserName;