Field Calculator

I'm trying to clean up some tables with the field calculator using python. Python is new to me so I'm not sure if it's possible. It would be nice if someone could post any page that can provide me with pytonsyntax for the field calculator.

This operation will only contain of cleaning up one field in each table. I want the field calculator to convert the records with
'x' to a NULL(None) value. There are also records with other strings too. I guess I will need if statement to convert other strings.

So how to make 'x' = NULL ?
Is it possible to convert all the strings to NULL values, not specifying the actual string?

I tried this:

def CleanField(InValue):
if InValue = 'x'
result = None
if InValue = 'y'
result = None
elif Invalue = Int
result = InValue
return results

All the records in the field where converted to NULL(None).

Just to mention it, the other values are numbers.