(2-part post):
Start by checking whether you're actually in an area which is currently expected to get digital reception. If you use the postcode checker, here, you may well find that the majority of channels are still 'greyed out' because (in theory at least) they won't be available until after the digital switchover is complete:
http://www.freeview.co.uk/availability/
In areas where the old analogue transmitters are still operating, the digital transmitters are currently operating on reduced power (in order not to interfere with analogue reception). Many people won't be able to get all digital channels until the analogue transmitters are switched off.
If you're in an area where the postcode checker suggests that you should get digital TV you need to check that you've got a decent aerial, such as this one
http://www.maplin.co.....aspx?ModuleNo=221124
AND that you've got modern, low-loss cabling.
If you're in a REALLY poor signal area you might also need a mast-head amplifier, such as this
http://www.maplin.co....e.aspx?ModuleNo=36573
which also need a suitable power supply, like this:
http://www.maplin.co....e.aspx?ModuleNo=44214