The above advice is about right, find publications/websites relevant to your business and send it to them, many will have a specific email address in their contact pages for press releases, or try and search out a direct mail for a relevant journalist.
One of the most important things though, is creating a good "hook" for your press release; people get tons of releases every day about new start ups, you have to try and really stand out and you do that best with an attention grabbing headline.
For example, I recently saw a good article on wildlife conservation, the press release had covered some fairly uninteresting data on fish stocks which showed that whilst a particular fish stock was stable, fish of a certain age within that species were at unusually low numbers, the headline was something like "fish could be extinct within a generation". It's a stretch of the truth but it's an attention grabbing headline and it got some dull research into the national press.
My first advice would be not to actually mail the release anywhere. Journalists won't read something that comes through the mail, it is too old fashioned. News moves so fast, it will feel like your announcement is old news by the time the mail reaches them. Some outlets, like the Associated Press, may accept faxes, but those end up on a news desks somewhere and often get lost in the shuffle.
Email should be your main form of contact. Like previous posters have suggested, build a targeted media lists of journalists writing in your area. A large majority will have an email address posted somewhere.
Good luck to you!