The easiest way to try 'PlusTalk' will really only require you to buy a headset.. This gives the microphone input and headphone output from your PC. This hardware can be had for under a tenner at such places as e-buyer. There is a Logitech Dialog 812 model that works well enough for the money.
You then install a 'softphone' on your PC which becomes your VoIP phone. You can make calls to PSTN at the call costs shown on the F9 site. You do not have to register for a new phone number. People can still call you on your old BT number (obviously) it won't ring your 'softphone' though.
You can register for a new number but I think callers get some 'non-geographical' type number that costs them more to call. We chose not to register a new number so we only use PlusTalk for outgoing calls.
You may have to open a couple of ports on your ADSL router to allow it to work - you can find the information on the F9 site.
Be warned that the call quality can be poor (see other thread). It isn't as bad as it was but it isn't as good as Skype IMHO.