I am going to send an email to a professor asking about her support for a postdoc position. I just want to know whether I attach my CV to my first email or it is better to wait for her feedback on my email and then send CV?
If you know they have a position, then yes. Otherwise it might be better to send a short note that is easy to reply to asking if they might have a position and offering to send any required information including a CV, etc.
This is a personal position, of course. I'd be more likely to respond to a short request and less likely to open or consider one that "floods the zone", unless I've advertised for candidates.
Your short note can just indicate your specialty in math and doctoral institution and date of degree. But even that is superfluous until you know they have a position.