If statement help

Hello,

What is everyone's favorite source for learning how to build if statements
and/or source of if statements. I am old to the term but new practical use
of these expressions.

More importantly, can someone help with this one:

I am an intermediate Assess user where I know tables, queries, forms and
reports inside and out with little knowledge in SQL statements and zero
knowledge in VBA.

I have a field that is manually N/A once a month based on what i put in
another field (service). the N/A field is a dropdown selection field based
on a member from the members table.

table is for duty assignment based on date (duties are dropdown selections
based on members table):
date: 13 mar 08 service: normal desert: fred
date: 15 mar 08 service: bla bla bla desert: n/a
date: 19 Mar 08 service: first desert:
george

so I won't have to add a bogus "N/A" member due to the field set to limit to
the list, is there a way, by if statement, to set it to populate that field
N/A if the service field is bla bla bla?

it only needs to be seen in the report and can not be left blank. I can
leave it blank in the form (table).

my best guess in layman's terms for the if/then logic:
if [service] is "bla bla bla" then N/A; where if [service] is anything else
then allow selected member

I know, funny huh?
Thanks in advance!