How to i advertise my building firm on the internet?

1) Make sure you've got it listed in the "Google Maps Local Business Centre"
http://www.google.com/local/add?hl=en&gl=uk

2) Make sure you get it listed in similar business directories aimed at the area you're aiming to work in, such as a website belonging to your local newspaper.

3) Buy a domain name + webhosting if considering knocking together a website. Doesn't have to be anything too complicated........ perhaps just as simple as details of how people can get hold of you, area you're prepared to work in, what tasks you're capable of doing (e.g. simple repair jobs / driveway work, through to knocking together a complete new building from scratch depending on what you mostly do) + a few pics of completed jobs you've done in the past.
For hosting, check out this company (they give out Â£400 worth of free software with their "1&1 Home" + "1&1 Business" hosting packages and above):
http://www.1and1.co.uk/?k_id=3794345
If you've never made a site before, it's pretty simple with the NETOBJECTS FUSION software included in the hosting package I suggested - http://www.netobjects.com
It makes putting together professional looking websites extremely easy.. but like I said, it doesn't have to be TOO complicated..... look how simple the website of BERKSHIRE-HATHAWAY (company owned by Warren Buffett, the world's 2nd richest man........ this company was valued at over $100,000 on the New York stock exchange last time I looked): http://www.berkshirehathaway.com/

4) If / when you've got a site going, try joining a few relavent Webrings @ http://www.webring.com

5) If necessary, follow it up with an advertising campaign on GOOGLE AdWords: http://pagead2.googlesyndication.com/pagead/iclk?sa=l&num=0&client=ca-ref-pub-5079162178336415&adurl=https://adwords.google.com/select/main%3Fhl%3Den_GB%26ai%3DBCB8yIVX_RZqrIISA-wLdneHxAZTmohXQ4pD9AcWNtwEAEAEg16u7BDgBUOyO_toBYLu-roPQCpgB8oYBmAGDhwGYAYaHAZgBnYcBmAG4kgaYAbuSBpgBvpIGmAGBkwaYAYWTBpgBj5MGoAHIvKf9A7IBFnd3dy53b3JtLWRhbmdsZXIuY28udWvIAQHaATFodHRwOi8vd3d3Lndvcm0tZGFuZ2xlci5jby51ay9odG1sL3JlZmVyZW5jZS5odG1sgAIBqAMD&ai=Bzjp2IVX_RZqrIISA-wLdneHxAZTmohXQ4pD9AcWNtwEAEAEg16u7BDgBULeZ-JP6_____wFgu76ug9AKmAHyhgGYAYOHAZgBhocBmAGdhwGYAbiSBpgBu5IGmAG-kgaYAYGTBpgBhZMGmAGPkwagAci8p_0DsgEWd3d3Lndvcm0tZGFuZ2xlci5jby51a8gBAdoBMWh0dHA6Ly93d3cud29ybS1kYW5nbGVyLmNvLnVrL2h0bWwvcmVmZXJlbmNlLmh0bWyAAgGoAwM