If you're under 20, you should also keep in mind that you have a zero alcohol limit. That means if you drive after consuming even one drink you can be charged with drink driving.
If you’re 20 or older, the legal alcohol limit is 250 micrograms per litre of breath or 50 milligrams per 100 millilitres of blood. Alcohol effects people differently, however, so if you have any doubt over whether you're safe to drive - don't. Hitch a ride with a friend, or call a taxi instead.
Check out the Road Code unit on alcohol and drugs for more information about blood alcohol limits and the penalties for drunk driving.