Is your garden a wild garden or a manicured garden? If its more wild and they don't bother you, just leave them. They are fascinating creatures.

If they do bother you and you want to get rid, you may have to speak to pest control.

Ants can do a useful tidying job in the garden since they assist in the breaking down of waste material, but they can become problematic if they invade pots & containers - although fitting the pots with "feet" should stop that from happening - and can also be hazardous if you have young children or people with a sensitivity to their bites or to formic acid.

There are a variety of "traditional" and extremely toxic insecticides that will kill ants, along with virtually everything else in the garden. I would strongly urge you not to consider using these as you risk damaging the whole ecosystem in your garden and poisoning a lot of essential garden animals.

Ecocharlie sell a new-ish Environmentally friendly product called No Ants:

http://www.ecocharlie.co.uk/nemasys-no-ants.html

These are nematodes that you water into the soil and which basically drive ants away from your garden.