I am a 20 year old British Male. I have all of my GCSE's (Standard qualifications in the UK) as well as:
-ONC in Basic Construction.
-HNC in Civil Engineering.
-4 years experiance with a Structural Engineers Firm in the UK.

I have friends out in Canada that run their own advertising company. They have said they are willing to give me job (effectivley meaning I have a job offer).

Where do I go from here? Who do I write to? What are my chances of getting a work permit for a year?

Thanks in advance to any response.

Im not entirely sure but from the sounds of it you might qualify as a skilled worker. Go to the offical canada immigration website and they have an online test that you complete, asks about qualifications, jobs etc to tell you if you qualify and the steps to take.

A Service Canada Confirmation would be required, by which the Canadian company would typically need to demonstrate that no Canadians were available for the position. With such a Confirmation, you could then see a work permit at a visa office or port of entry.