I presume you've got a gmail account. If not go onto gmail and create one.
Then go to
https://www.google.com/accounts/ServiceLogin?service=lbc&continue=https://www.google.com/local/add%3Fservice%3Dlbc and log on with your gmail account.
Fill in details of your business and submit (has to have a valid postal address).
After 10 days or so they will send you a postcard with a PIN no. Log in again (details on the postcard) and give the PIN no (this allows them to verify that you live at the stated address).
Within 24 hours you'll be on the front page of Google. Then type your postcode into Google maps and scroll to the bottom of the listings to the link to add your company name on the map.
All free. (Well, sort of free. Everyone forgets that we are 2 billion worldwide paying £20 a month to be online. I bet George Osbourne wouldn't turn his nose up at that

)