I need a text field to show NA if a numeric field is Null and then ""True"" if it is 0

try this

IF(
IsNull([my field]), "NA",
[my field] <0, "True",
[my field]>0,"False",
[my field]=0, "I'm not sure what you want this to be")


note: with numeric fields, null and zero and not necessarily the same.  It depend either on the formula that calculated the result or if its a data entry field, then how the field properties are set for how to treat blank.

Thank you!