ResultSet.getString(1) throws java.sql.SQLException: Invalid operation at current cursor position

You should use the next statement first.

ResultSet set = statement.executeQuery();
if (set.next()) {
    userName = set.getString(1);
    //your logic...
}

UPDATE

As the Java 6 Documentation says

A ResultSet cursor is initially positioned before the first row; the first call to the method next makes the first row the current row; the second call makes the second row the current row, and so on.

This means when you execute the sentence

ResultSet set = statement.executeQuery();

The ResultSet set will be created and pointing to a row before the first result of the data. You can look it this way:

SELECT email,firstname FROM registrationinformation

    email              | firstname
    ____________________________________
0                                        <= set points to here
1   [email protected]   | Email1 Person
2   [email protected]        | Foo Bar

So, after openning your ResulSet, you execute the method next to move it to the first row.

if(set.next()) 

Now set looks like this.

    email              | firstname
    ____________________________________
0
1   [email protected]   | Email1 Person   <= set points to here
2   [email protected]        | Foo Bar

If you need to read all the data in the ResultSet, you should use a while instead of if:

while(set.next()) {
    //read data from the actual row
    //automatically will try to forward 1 row
}

If the set.next() return false, it means that there was no row to read, so your while loop will end.

More information here.