using position in where

I have an application where I need to "select" based on whether or not a
"text" column value contains a given substring. I have tried the "position"
function as follows, but it doesn't return anything:

select * in customers where position ('sub_string' in 'text_column') > 0;

Is there another way to do this?

Another question - how are upper and lower case handled when using "order
by"? In my experimenting, it seems to be doing a case insensitive compare,
but the docs I've read seem to indicate otherwise.

Thanks in advance,
Lynn