If you can receive Freeview through your existing TV aerial now, then you should be OK after the digital switchover. After switchover the signal strength will be increased and you should get an improved picture.
Freeview HD is only available after the switchover. You will need a Freeview HD box or have a TV with a Freeview HD Tuner built-in. If you are moving to a new home, (which normally do not come supplied with TV aerials!) it would be best to get a high Gain aerial. Your installer will be able to advise. Expect to pay up top £150 for a new aerial or upgrade.