http://www.digitaluk.co.uk/when_do_i_switch
If you are still receiving Analogue TV then your region has not yet switched to Digital. Check above site to find out when and enter your PostCode for more details.
There is no such thing as a Digital Aerial as your existing Aerial, providing it is in good condition should suffice.
If you live some considerable distance away from your transmitter and the reception is not ideal, then a WideBand TV Aerial may be necessary along with either a MastHead Amplifier or an indoor one.
The quality of the Freeview Boxes that you buy have a bearing on what you are able to receive, as some of the inferior quality ones have ceased to function as they cannot cope with the amount of Channels which are now available. I have encountered 1 integrated Philips TV (Part Failed)and 1 Matsui Freeview Box (Failed Completely)in the last week,whilst doing Full Re-Scan.
Check the Digital UK Site first to see how your situation is regarding Freeview reception and take it from there.