How do I activate my payment gateway?

[saksirisan]

I have installed Virtuemart - now what do I do to accept credit cards / paypal?

Do I have to setup an account with authorize.net and then what do I do?

Also, When I have Store Credit Card information, is that stored on my system or Authorize.net? Is that safe?

Lastly, I some additional modules that are supposed to increase security and reduce chargebacks, etc - are those necessary?