Blackberry usually polls for email every 15 minutes. I am on o2 using a yahoo email address and my email comes almost instantly.
o2 will also supply you with an o2 email address that you can use if you wish, I don't use it as I am happy with Yahoo.
Here is a link to o2's Blackberry faq page which you might find helpful, it includes instructions on how to set up email.
BlackBerry from O2 - FAQs
These links might be helpful too.
BlackBerry from O2 - Getting Started
Methods of Receiving Email on BlackBerry - BlackBerryFAQ