How can i tell how much data i am using a month? Is there a section on the bt website that i can log onto and check?

Your hub home page will have the upload and download usage.
It is one of the hubs features.

All hubs have the same address.

Three ways (i know) to get to your home page ..

http://bthomehub.home/

Or type this in to your internet address bar at the "Very top of the page " you know the one that
say http//uk.answers ( not the search box)

192.168.1.254

Or make sure your network discovery is on then look in your network.
and click on the icons ,one of which will be your router.

If you would like any more info on the BT hub i have found this site of great use.

http://www.frequencycast.co.uk/homehub.html

I have a headset with microphone as used in internet cafe’s plugged into my pc.
How do I use the internet on my pc connected to bt broadband as a free telephone service?

Just connect a digital phone to your BT Hub…freephone calls depend on which option you’re on…check your contract
usually evenings after 6pm….and weekends are free to landlines….you have to pay for calls to mobiles…again check contract
If as has been suggested, you want to use SKYPE….theres 2 options…(1) SKYPEOUT..costs you about 1.4 pence per minute to landlines..local and international
(2) SKYPEIN….calls are free BUT both you & other party have to buy a yearly subscription…costs about £10-15 per year
so unless you intend to make a lot of calls i think you’re cheaper with SKYPEOUT…its up to you..just read all the bumpf before you decide…hope thats a help

I have a dell pc which is connected by ethernet to my bt home hub. I now another pc upstairs and would like to connect that to my bt home hub but have no phone line upstairs. Can i use a wifi thing and if so what are they called and which one is best/where can i buy? Thanks for any relevant info!

Yes you can, i have done that myself with the bt home hub.