Selecting fields that don't have any values in them

[Pathfinder]

Likely a totally dumb question, but I have a field in my database called "Paid". I didn't set the default of this field to NULL when I created the database (dumb, I know now) but now I only want to pull the records from the table in which nothing has been entered into the Paid field.

I have a feeling that maybe isset has something to do with it, but don't know how to incorporate it into my code, which is:

$sql = 'SELECT FirstName,Paid FROM test WHERE PaidIS NOT NULL LIMIT 0 , 30';

And doesn't WORK, waaaaaa

I know it's staring me right in the face, maybe you can be kind enough to lighten the way for me ...

Thank you,
Pathfinder

[Frederick]

$sql = mysql_query("SELECT * FROM tablename WHERE Paid < '1' ");