How can i advertise my company for free?

A lot of this depends on what your company provides and who the people are who are most likely to buy what you are selling, but here are some general ideas.
Go to the business section of the nearest public library and talk to the librarian. He/she should be able to point you to a number of resources. The book, "Guerrilla Advertising," comes to mind. You should be able to find several books talking about no cost or low cost advertising.
While you are there, look at books about public relations as well. (Look in most general interest magazines, and there is editorial coverage of different goods and services. That didn't cost the maker of those goods and services anything.)
If you are really dedicated, you can learn to write press releases and do this yourself to interest local or regional or national coverage, or you can hire a p/r person. They should be able to generate big multiples of their fee in exposure. (In other words, for every $1000 of fee, they should be able to get you $3000 to $10,000 of free exposure if you measure what the same exposure would have cost you in paid advertising.)
Join a business group. There are several variations of the idea, small business people get together for breakfast on a regular basis, trade business cards and network among themselves.
Put a line listing in the local phone book.
Offer your product or service to local charities for them to auction for fund-raisers. They will advertise your company among people with disposible income.
Start a blog giving advice that leads to your goods and services, and establishes you as an expert whatever-you-do.
Partner with complementary businesses. For instance a rug store in my town has a few pieces of furniture just to show how it will look with the rugs. The furniture came from another local store and is on loan. There is a large framed ad on a table saying the furniture came from there. At the furniture store are a few rugs from the first store with another framed ad for the rug store. Both change out stock every month or so.
There is also a restaurant that has special breads made by a well known local bakery. There is a sign at the front of the restaurant saying so. At the bakery is a sign suggesting the restaurant for catering.
Once you really start to think about it, you should be able to come up with more ideas. The trick is finding ideas that are effective, but don't require a lot of your time to implement.