They are ready when the shoots start to go yellow/brown and wilt.
Once harvesting you have 2 choices, eat straight away or allow them to dry. If you choose to eat straight away (known as wet garlic) then its simply to the kitchen, wash and use, theres nothing else to do. For dried garlic you simply wash and allow to dry out in the sun, its ready when the skin is dry i.e. the garlic you buy from the supermarket. There is a noticable taste difference between wet and dry garlic, we cook some wet and allow some to dry, I prefer wet since it has a stronger sweeter taste.
What you will most likely find is that it wont have the cloves, you need a frost on the plants to get them to split like that, it'll probably be like a large sping onion.
<edit>We get our garlic from this place, its local to us.
http://www.thegarlicfarm.co.uk/
Nows a good time to plant garlic overwinter ready for the spring.