I could go into more detail, Smow, but I'd bore you rigid and I have an appointment to keep!
If you don't run, you might start with a gel heel cup (You might be better off lifting up the insole and put the heel cup underneath - they can feel a little odd on bare feet). Or you can go to a full insole that will have gel in the heel AND forefoot. If you do run, you might have to go a specialist shop and have your GAIT analysed. If you overpronate (Your arch collapses - think of walking behind a girl wearing UGG boots and the soles are at a funny angle, almost like her arches are touching the tarmac) you might need a different type of shoe (Adidas Response Stability, Asics Gel Virage, Asics GT2170 or a Nike shoe with DYNAMIC SUPPORT stamped along the midsole).
Again, if you do run, you might have to take it easy for a couple of weeks to let any inflammation or pain subside before you start up again.