Perform SQL Query WCF RIA Silverlight

I have created a database, linked it with DomainService's within my Silverlight Application. Now I want to be able to perform certain actions, such as Registration, Login etc. by using the service.

How would I be able to do this. I have preset methods created in the service, e.g. InsertUser but it only requires one parameter, so I'm not sure how it works. In the metadata I have all fields etc.

Can anyone help me out here.

Thanks.

public IQueryable<User> GetUsers()
        {
            return this.ObjectContext.Users;
        }

public void InsertUser(User user)
        {
            if ((user.EntityState != EntityState.Detached))
            {
                this.ObjectContext.ObjectStateManager.ChangeObjectState(user, EntityState.Added);
            }
            else
            {
                this.ObjectContext.Users.AddObject(user);
            }
        }

For retrieving User I have used (as a base from TBohnen.jnr code):

UserContext _userContext = new UserContext();

        public MainPage()
        {
            InitializeComponent();
            LoadOperation loGetUsers = _userContext.Load(_userContext.GetUsersQuery());
            loGetUsers.Completed += new EventHandler(loGetUsers_Completed);
        }

        void loGetUsers_Completed(object sender, EventArgs e)
        {
            LoadOperation<Web.User> lo = (LoadOperation<Web.User>)sender;
            var user = _userContext.Users;
            MessageBox.Show(user.ToString());
        }

This is to add a new user:

YourDomainContext dc = new YourDomainContext();
User userToAdd = new User();
//You will have to set your properties here as I don't know them, I will give an example.
userToAdd.username = "NewUser"; 
dc.User.Add(userToAdd);
dc.SubmitChanges();

To retrieve your existing users:

YourDomainContext dc = new YourDomainContext();
LoadOperation loGetUsers = dc.Load(dc.GetUsersQuery());
loGetUsers.Completed += new EventHandler( loadOperation_Completed );// You will see there is a callback overloads as well

and then add this as well.

private void loadOperation_Completed( object sender, EventArgs e )
{
    LoadOperation<User> lo = (LoadOperation<User>)sender;
    //Have a look at all the properties like lo.Error etc. but to see the retrieved users you can either use:
    var users = lo.AllEntities;
    //or if you declared your domaincontext as a class level parameter:
    var users = dc.User;
    foreach (Web.User user in users)
    {
        MessageBox.show(user.username);
    }
}

This will trigger an async call that get's all the users and will add it to the DomainContext and you will be able to access it through dc.User