[SQL] Query Help

[Brian C. Doyle]

What do I have to do a query where information in table1 is not in table2

I am looking for something like

Select table1.firstname where table1.firstname is not in table2.firstname 
and table2.date='yesterday'

I tried
Select table1.firstname where table1.firstname != table2.firstname and 
table2.date='yesterday'

and that did not work just listed everyone....